Xi Jinping came to power in 2012 amid a serious foreign policy crisis following the Japanese government’s nationalization of three of the disputed Diaoyu/Senkaku Islands. The move was aimed at blocking Shintaro Ishihara’s Tokyo municipal government from purchasing the uninhabited islands from their private owner and constructing a harbor and other infrastructure that would further inflame tensions. Although Beijing understood Prime Minister Yoshihiko Noda’s goal was to prevent an even worse crisis, it nonetheless responded by sending maritime law enforcement ships into the Japanese-controlled territorial seas around the islands, backed by a furious propaganda wave that unleashed violent anti-Japanese protests in dozens of cities across China. The result was a fundamental change in the sovereign territorial seas around the islands from Japanese control to overlapping control — a situation that continues to bring ships and aircraft from both sides into close contact on the water, increasing the risk of an accident that could bring about armed conflict.

How much of China’s behavior in the East China Sea is attributable to Xi Jinping? To what extent have Sino-Japanese action-reaction dynamics been at play versus longer-term processes of the shifting distribution of power in the region or changes in domestic politics? How important have the hydrocarbon deposits that originally touched off the dispute been, and to what extent have the area’s fisheries — or fisherfolk — been protagonists? This paper assesses these questions using a systematic time-series dataset of both sides’ patterns of behavior over the long term, from its origins in the early 1970s to the Xi era. It visualizes the historical trajectory of the Sino-Japanese disputes in the East China Sea, charting each side’s key moves across domestic, diplomatic, and physical domains and the balance between military, administrative, political, and resource motivations behind them. Quantifying the changes in China’s and Japan’s behaviors in the disputed area from 1970 to 2015 reveals six key dynamics:

1. The East China Sea dispute began over oil and gas resources but switched toward a contest for military and administrative control as China rapidly expanded its naval and coast guard presence in the mid-2000s.
2. China’s policy was already trending in an increasingly assertive direction well before Xi took power. China’s gray-zone assertiveness dates back to the mid-1990s, while coercive methods started in the mid-2000s. The key change Xi has overseen is China’s increasingly militarized — but also regularized — presence in the disputed area.
3. Japanese actors have triggered several acute periods of tension with provocative moves, but China has driven the long-term arc, with its shifts from “shelving” the dispute in the 1970s to greater assertiveness from the mid-1990s to regular coercion from the mid-2000s.
4. The two significant periods of non–Liberal Democratic Party rule in Japan have both preceded surges of Chinese assertiveness followed by Japanese pushback, raising questions about China’s calculations regarding domestic politics in Japan.
5. Despite several high-profile propaganda campaigns and diplomatic blitzes, most of China’s moves have been in the physical domain on the water, while Japan has focused on diplomacy and domestic administrative moves.
6. Xi’s precise role in the escalation around the disputed islands in September 2012 remains unclear, but his centralization of power since the 18th Party Congress has coincided with a regularization of China’s assertive behaviors. A less powerful leader might, like Xi’s predecessors, find it more difficult to prevent substate actors from taking destabilizing actions in the area, as occurred several times in the 2000s.

The East China Sea disputes concern territorial sovereignty over the Diaoyu/Senkaku Islands, together with the fisheries resources of the surrounding waters and hydrocarbon reserves beneath the seabed. Sino-Japanese disputes in the area date back to 1970, when the People’s Republic of China (PRC) staked its first explicit claims. Well-known episodes of acute friction over the issue have sent the bilateral relationship into freefall on several occasions. In 2010, a drunken PRC fishing boat captain rammed a Japanese Coast Guard vessel, touching off a tense standoff in which China constricted important rare earth shipments to Japan. In 2012, the two countries lurched toward the brink of armed conflict after Japan nationalized three of the islands, prompting China to start sending its maritime law enforcement ships into Japan-controlled territorial seas. Japanese public perceptions of China have never recovered from the scenes of destruction wrought by violent nationalist protests over the issue in August–September 2012.

The islands at the center of the dispute sit approximately 105 miles northeast of Taiwan, “where the water turns black,” as Chinese fishermen have traditionally described it, and the continental shelf of Eurasia meets the Okinawa Trough. In contrast with the tiny, low-lying disputed coral atolls of the South China Sea, the Diaoyu/Senkakus rise out of the sea as a series of peaks on an underwater mountain range. They have an area of 2.7 square miles and are home to a variety of bird species, feral goats, and a rare mole but have been uninhabited since a short-lived Japanese fish cannery failed in the 1940s. After World War II, U.S. forces occasionally used the islands as a bombing range before transferring administrative control to Tokyo in 1972 as part of the Okinawa Reversion — though without taking a position on their sovereignty.

The dispute first rose to prominence after UN-sponsored preliminary surveys in 1968 indicated the potential for large deposits of oil and gas under the East China Sea. This potential bounty prompted both Mao’s PRC and Chiang Kai-shek’s Republic of China to assert China’s sovereignty over the islands, arguing that they had been ceded by the Qing Dynasty as spoils of war along with Taiwan and Penghu in 1895 and thus should have been returned to China at the end of World War II. While the island dispute has prevented comprehensive surveys across many parts of the sea, China has set up oil and gas drilling platforms in waters further to the north. These installations are subject to a further layer of dispute, with Japan arguing they tap into deposits that straddle the median line demarcating its claimed exclusive economic zone and that China’s production activities therefore siphon resources from the Japanese side. Today, the United States Geological Survey estimates the area’s oil reserves at between 0.3–1.7 billion barrels of petroleum and 1.3–7.3 trillion cubic feet of natural gas.

The seas around the Diaoyu/Senkaku Islands have long been important fishing grounds for communities in northern Taiwan, the Ryukyu Islands, and mainland China. In fact, “Diaoyu” (or “Tiaoyutai” in Taiwan) means “Fishing Islands.” Although subject to overlapping claims, the fisheries resources have not always driven friction. In 1997, China and Japan signed a cooperative agreement enabling reciprocal access for each side’s fisherfolk and creating a joint management scheme; a similar agreement between Taiwan and Japan was signed in 2013.¹

Given the contentious 20th-century history between the two countries, including Japan’s invasion of China during World War II, the islands hold particular political significance in China as a symbol of Japan’s alleged unrepentance and ongoing aggression. On the Japanese side, meanwhile, the islands are a focal point for concerns about the seaward expansion of China’s military and administrative power. As a result, tensions continue to simmer, such that any unexpected incident or accident in the area carries the potential to touch off a major crisis. Against this backdrop, avoiding unintended escalation and controlling incipient crises requires understanding the long-term trajectory and drivers of China’s and Japan’s behaviors in the East China Sea dispute.

This paper presents a parallel time series of China’s and Japan’s assertive moves in the East China Sea dispute from 1970 to 2015. In total, the dataset captures 74 assertive moves by China and 46 corresponding moves by Japan. The data were assembled, with support from Lancaster University, as part of the Maritime Assertiveness Time Series (MATS) project, which seeks to quantify state behavior in maritime disputes in East Asia and beyond. This ongoing project began by comparing China’s, Vietnam’s, and the Philippines’ behaviors in the South China Sea, the data for which are visualized on the Maritime Assertiveness Visualization Dashboard developed and hosted by the National Bureau of Asian Research. This report expands the scope of the project to cover the main protagonists in the East China Sea.

Cases enter the MATS dataset through a two-stage process. The first stage is the generation of master timelines into which any potentially relevant historical developments in the dispute are entered. Sources include Chinese-language materials such as PRC government reports and maritime agency yearbooks, Japanese government statistics on Chinese activities, U.S. State Department cables, and historical works by foreign-based journalists and academics. After casting a wide net in the first stage, the second stage involves refining and coding entries from the master timeline that meet the definition of assertive activity, as discussed below.

“Assertiveness” has arguably defined the English-language discourse on PRC maritime policies, but the term’s precise meaning has often been fuzzy. Although it has sometimes been synonymous with “coercion” and “aggression,” the term covers an array of methods, both verbal and physical — though not necessarily directly confrontational — by which states pursue their interests during a dispute. A useful dictionary definition is “bold or confident statements and behavior,” which, in the context of maritime and territorial disputes like those in the East China Sea, covers statements and behaviors that advance or strengthen a state’s position in the dispute. This definition breaks assertiveness down into observable events — statements and behaviors — that can be counted and categorized over a given time period.

However, in active disputes like the East China Sea, statements and actions are occurring on a near-daily basis, making it challenging if not impossible to capture and count all of them. A more feasible goal is to capture the dynamics of state behavior by identifying assertive moves (i.e., changes in a state’s behavior from one time period to the next). Assertive moves are identifiable when a state’s observed actions constitute a new method of advancing the claim unseen in previous time periods; are more frequent than in previous time periods, such as an increase in patrol activity or resource exploration and exploitation; or are applied over a broader geographic area than in previous time periods.

Cases that enter the dataset are then coded by type, issue, and domain. Assertive moves fall into four broad types based on their increasingly serious implications for the positions of rival claimants: (1) declarative, (2) demonstrative, (3) coercive, and (4) use of force. Distinguishing these four qualitatively different categories of assertive state moves in maritime disputes makes it possible to identify changes in the quantitative level and qualitative type of assertiveness in a state’s behavior across time. Cases were also coded according to the specific issues on which a state focused, such as military control, civilian administration, energy, fisheries, and political support, as well as the domain of contestation, such as domestic politics, law, and administration; international diplomacy; or physical contestation in the disputed area.

As summarized in the table below, individual assertive moves belong in the highest-level category for which they meet the criteria. Thus, patrolling a disputed area is classified as a demonstrative behavior, even though it may also involve declarative verbal proclamations of the state’s claim to the area. Likewise, direct interference with another state’s construction project or resource survey belongs unambiguously in the coercive category, even if it also entails declarative and demonstrative elements. Direct seizure of a particular disputed land or sea area constitutes a use of force, even though it usually also involves buildups of administrative presence (demonstrative), threats of punishment (coercive), and verbal claims (declarative).

What can the maritime assertiveness framework tell us about the long-term patterns of behavior in the East China Sea and its more recent drivers? The fundamentals are illustrated in Figure 1, showing the number and type of assertive moves by China and Japan in the dispute each year since 1970. Declarative moves are shown in white; demonstrative moves are shown in gray; and the sections depict the yearly number of coercive moves. No full-blown use of force — military attack or seizure of territory — has yet been observed in the East China Sea.

As the chart illustrates, the East China Sea maritime dispute first flared up in 1970 following the publication of preliminary surveys suggesting significant oil and gas deposits. The first assertive state actions were on paper, with a Japan–South Korea agreement for joint development of the promised bounties, which prompted protests from both China and Taiwan. With no immediate prospect of extraction and momentum toward the normalization of Sino-Japanese relations after 1972, tensions receded until 1978. But ahead of the normalization of bilateral relations, both sides felt it necessary to demonstrate their resolve: China with a show of force by an armed fishing militia, and Japan by permitting far-right group Nihon Seinensha (Japan Youth Society) to erect a lighthouse. As the chart illustrates, ironically, the dispute was only dormant in the six years before Deng Xiaoping’s 1978 statement that it should be put off and left to future generations to resolve.

In the 1980s, despite the dispute being officially shelved from the PRC perspective — and nonexistent from Japan’s — assertive moves were an increasing occurrence. Indeed, this dormant status itself became a subject of dispute in May 1989, when reports emerged that the Japanese foreign minister had told Soviet leader Mikhael Gorbachev that there was no dispute over the islands, prompting the PRC Foreign Ministry to publicly reassert China’s “indisputable sovereignty since ancient times.” The following year, tensions came to a head over Japan’s reported plans for formal government recognition of the Nihon Seinensha lighthouse, which the group had renovated in 1987–88, and its interception of Taiwanese protesters seeking to land on the islands — its first known deployment of physical coercion on the water around them.

Assertive moves became much more common through the 1990s, with only one quiet year, in contrast to the 1980s, when six of ten years passed without an observed change in claimant behavior. A particularly serious set of actions and reactions occurred in 1996, following the two countries’ ratification of the UN Convention on the Law of the Sea, which formalized their overlapping claims to the East China Sea’s energy and fisheries resources. Nihon Seinensha returned to the disputed islands to build another lighthouse, attracting bitter condemnation from China, including the first direct accusation that the right-wing youth group’s actions were “not accidental and are directly related to the Japanese government’s attitude.” Tokyo responded by making its tolerance of the activism explicit, with Chief Cabinet Secretary Kajiyama Seiroku stating that Japan could not approve or disapprove of building a lighthouse on privately owned property. When anti-Japanese activists from Hong Kong sailed to the islands to protest, one perished in an attempt to swim ashore, bringing tensions to a boiling point. Fearing instability, Beijing tamped down a nascent protest movement by nationalist students on the mainland but also made a substantive show of force by sending military ships and aircraft into the area.

As the chart makes clear, China’s pattern of regular assertive advancements in the dispute can be traced back to the mid-1990s, and they accelerated with the introduction of regular coercive behaviors — shown in red — around a decade later. In the first 25 years of the dispute, China made 11 identifiable assertive moves (0.4 per year), whereas there were more than 60 in the next 20 years (3 per year). The step-change was mirrored on the Japanese side, with only 10 assertive moves (0.4 per year) from 1970 to 1995, but 1.75 per year after 1996. China’s protracted, patient efforts to change the status quo in its favor through unilateral administrative moves — the gray bars — date back to the 1990s. The red-colored bars, meanwhile, indicate the introduction of regular coercive moves involving the threat or imposition of punishment, particularly since 2004. Both shifts were observed on the PRC side first, with similar changes in Japan’s behavior following soon after, suggesting the broad patterns of China’s gray-zone assertiveness and coercion are attributable neither to Japanese provocation nor to Xi Jinping.

The timing of behavioral changes in the East China Sea does suggest an intriguing link with domestic Japanese political developments. Since 1955, the Liberal Democratic Party has usually been the majority party in Tokyo. The two exceptions have been 1993–95, during which Prime Minister Murayama made Japan’s most explicit apology for past atrocities in the region, and 2009–12, when the Democratic Party of Japan (DPJ) came to power following Yukio Hatoyama’s victory in the 2009 election. Both of these periods of relatively China-friendly governments in Japan preceded surges in Chinese assertiveness in the East China Sea. In the most recent period, PRC assertiveness prompted a dramatic hardening of the DPJ government’s handling of the issue, empowering its most China-skeptic elements to take charge of Japanese policy and precipitating unsteady crisis responses, such as a fishing boat incident in September 2010. This suggests a possible miscalculation on China’s part regarding the state of domestic politics in Japan and raises the question of whether its behavior again may change in response to Japan’s hung parliament elected in October 2024.

In sum, the data illustrate that Chinese policy was already trending in an increasingly assertive direction well before Xi came to power. China’s gray-zone assertiveness dates back to the 1990s, while its regular use of coercive methods became a feature in the mid-2000s. Corresponding increases in Japan’s assertiveness followed after, not before, China’s key changes in behavior. Thus, there is little basis for attributing these fundamental aspects of China’s conduct to either Xi Jinping or to changes in Japan’s behavior. They are, instead, well-established policy directions that Xi has carried forward or deepened. The next section examines what the historical data suggest has driven the two sides’ assertive maritime conduct. 

The issues surrounding the East China Sea disputes are multi-layered, and the assertiveness data provide insights into the changing nature and focus of the disputes over time. Has the primary driver been the hydrocarbon deposits that originally touched off the dispute, or did it long ago become a struggle for military control? To what extent have the area’s fisheries — or fisherfolk — been protagonists? To what extent has the dispute become, as Todd Hall has argued, fundamentally symbolic and political rather than material?

Figure 2 shows the issues that China’s and Japan’s assertive moves have been directed at in each year covered by the database. Military moves oriented toward obtaining an advantage in the event of armed conflict (or denying the same to an adversary), such as naval movements, weapons deployments, exercises, and confronting other militaries in the area, are shown in black at the top. Administrative moves, shown in purple, are behaviors expanding or intensifying the state’s civilian administrative capacity or presence in the disputed area. The proportion of assertive moves oriented toward domestic or international political support is shown in green, while the proportion of resource-oriented moves is shown at the bottom of the chart in yellow (energy) and red (fisheries). Many assertive moves serve multiple purposes, so cases can belong to more than one issue area.

The chart shows a clear contrast between the issues toward which China and Japan have directed their assertive efforts. As the yellow blocks indicate, the touted hydrocarbon bounties that originally touched off the dispute in 1970 were a prominent factor in around half of China’s assertive moves until the mid-2000s. From that time forward, however, the PRC focus has broadened to building up its overall military and administrative presence, notably through increases in patrolling activity. By comparison, Japan has been far less active in pursuit of the area’s energy resources, though it resumed its protests against China’s unilateral energy exploitation projects in 2015 after several years of silence.

The charts illustrate that tensions over fisheries produced only rare and isolated incidents until around 2006. Since that time, however, Chinese and Taiwanese fisherfolk began featuring more often. The infamous 2010 ramming of a Japanese Coast Guard vessel that sparked a bilateral crisis emerged from a lesser-known backdrop of unprecedented numbers of PRC vessels engaged in coordinated fishing in the seas around the disputed islands — a pattern that has been repeated in some years during the Xi era. However, in most years — most recently 2024 — local authorities have restrained Chinese fishers from sailing into the disputed waters. It is possible that Xi’s centralization of power has enabled the effective implementation of such control measures. While China’s military activities have generally been tightly controlled and the China Coast Guard’s expanded patrolling presence was centrally mandated in 2006, the Hu Jintao and Jiang Zemin administrations at times struggled to prevent substate actors, including fisherfolk, from stoking Sino-Japanese tensions.

China and Japan have also focused on significantly different domains, as illustrated in Figure 3. Since the current pattern emerged around 2004, the overwhelming bulk of China’s assertive changes in behavior have occurred in the physical domain (blue bars), directly advancing its claim through activities in the disputed area. Japan, in contrast, has more often attempted to consolidate its position through a series of domestic institutional and legal moves (black bars) and extensive diplomacy-seeking coordination and support from foreign actors (red bars). This observation suggests the shift observed in the mid-2000s marked the beginning of China’s attempts to challenge and gradually overturn the status quo of Japanese control of the disputed islands and surrounding waters, established with the Okinawa reversion in 1972. Understanding the disputed islands as under its control, Tokyo primarily responded with new domestic and diplomatic moves. For China’s part, despite loud rhetoric and propaganda campaigns during crises, its efforts in the diplomatic domain have not kept pace with its new and expanded activities on the water during the same period.

The charts above illustrate how the East China Sea dispute began over oil and gas resources but switched decisively toward a contest for military and administrative control as China rapidly expanded its naval and coast guard presence starting in 2006. While Xi has maintained existing lines of effort, he has also overseen a rebalancing toward military objectives — both in the air and on the water. This has ranged from new patrol and reconnaissance activities by the People’s Liberation Army (PLA) Navy at increasingly close distances to the disputed islands to close-in aerial intercepts of Japanese planes by the PLA Air Force to arming China Coast Guard patrol boats with naval artillery. In short, neither the growing PRC presence in the disputed area nor its increasingly coercive character are particular features of the Xi era — rather, the key differences have been the increased militarization and regularization of that PRC presence.

The observation that PRC policy was already trending in an increasingly assertive and coercive direction before the Xi era carries three clear policy implications. First, it implies a different Chinese leader would be unlikely to result in a major change in China’s behavior in the East China Sea dispute. Rather than reflecting the particular policy preferences or risk appetite of individual leaders, the long-term data presented here suggest the assertive and coercive outlines of China’s behavior instead reflect well-entrenched policy settings.

Second, the data provide little support for China’s contention that its policies in the East China Sea have been mere defensive responses to Japanese aggression. Japanese actors have triggered several acute periods of tension with provocative moves, most notably in 1990, 1996, and 2012. However, the evolution from shelving the dispute to greater assertiveness and regular coercion began on the PRC side on the basis of its expanding maritime capabilities, with Japanese behavior following suit in subsequent years. The timing of at least some of China’s moves regarding Japanese domestic politics — in particular, a belief that more dovish administrations in Tokyo would tolerate expanded PRC activities — could offer a partial explanation.

Third, paradoxically, Xi’s centralization of political power may also have rendered PRC policy more predictable and perhaps at times even restrained (e.g., by preventing large-scale fishing expeditions in the East China Sea’s “sensitive sea areas,” expansions of oil and gas exploitation, and incidents like the September 2010 trawler incident). While Xi’s exact role in the 2012 escalation around the disputed islands remains unclear, his centralization of power since the 18th Party Congress has coincided with the regularization of key aspects of China’s assertive behavior in the dispute, such as its maritime patrolling activity in the territorial seas around the Diaoyu/Senkaku Islands and the PLA Air Force’s overflights in the East China Sea. A less powerful future Chinese leader, like Xi’s predecessors Hu Jintao and Jiang Zemin, might find it harder to prevent substate interests and social actors from generating greater unpredictability in Sino-Japanese relations.

Acknowledgments: The author thanks two anonymous reviewers for their helpful feedback, and the CCA team, particularly Jennifer Choo, Ian Lane Smith, Clara Lambert, Jing Qian, and Lyle Morris for their support of the project. Tyrique Mowatt and Olivia Timbrell provided outstanding research assistance as part of a summer internship program in the Department of Politics, Philosophy and Religion at Lancaster University. The author bears sole responsibility for all errors and shortcomings. 

1. While the energy and fisheries resources can be roughly estimated, the islands’ value is much more difficult to quantify. The location is strategic for its proximity to both the “First Island Chain” and the busy sea lanes that carry up to 35% of global petroleum shipments, upon which the economies of China, Japan, and South Korea depend.