The East China and South China Seas have long been cast as twin problem spots in the Asia-Pacific security landscape. Ensnared in complex and baggage-laden histories, both disputes have seemed equally intractable and have also been focus points for Beijing to flex its burgeoning military and coercive-diplomacy muscle. All observers expected tensions to keep rising in both disputes as China continues to build up its capabilities and brandish its hardened diplomatic resolve.

But the last year has seen the disputes evolve in dramatically divergent ways. Tensions have dropped perceptibly, if not significantly, over the East China Sea. Unplanned encounters between boats and aircraft have decreased and China's Air Defense Identification Zone (ADIZ) has not substantially hampered routine air traffic over the disputed area as many feared when the ADIZ was initially declared. The leaders of China and Japan have even held two terse face-to-face meetings that nonetheless broke a long-time freeze in high-level official interaction between the two sides.

Meanwhile, tensions in the South China Sea have flared. Though a low hum of troubling incidents have afflicted the region for years, international attention has recently focused on accelerated Chinese efforts to reclaim land in disputed areas of the South China Sea, turning atolls into bona fide islands that now house facilities and equipment (such as runways and docks) with potential military applications. Signaling and rhetoric has grown bellicose, particularly between the U.S. and China; the U.S. recently conducted a reconnaissance flyover of some of these man-made islands with a CNN crew in tow to broadcast their findings. This prompted the party-run Global Times newspaper to flatly warn the United States that "war will be inevitable" unless the U.S. gets out of China’s way in the South China Sea.

Why is one dispute simmering down while the other is heating to boiling point? These developments defy the usual logic that analysts have applied to the disputes—whereas China previously seemed bent on wielding its growing power wherever it claims "core interests," it now seems to be applying its power with greater discretion. If one is to lend credence to the idea that the Chinese regime uses the maritime disputes to consolidate domestic support by stoking nationalist sentiment, then the ongoing thaw in China-Japan relations, particularly on the much-trumpeted 70th anniversary of the end of World War II, is especially puzzling. Some observers have suggested that Chinese President Xi Jinping is pulling back in the East China Sea because he now feels sufficiently secure in his power; if that is the case, then why have we not seen any such easing on the South China Sea or any of his other putative “power-consolidation” tools, such as his domestic ideological hectoring, crackdown on speech or even the anti-corruption campaign?

The key factor at play is China’s position in the two disputes; its hand is much weaker in the East China Sea than in the South China Sea. Since placing a sovereignty marker on the Senkaku/Diaoyu Islands in 1895, Japan has exercised far greater de facto control over the islands than China, which gives it an immense advantage in the sovereignty dispute with China. With no opening to gain a toehold of control in the islands, China faces an uphill battle to establish its sovereignty claims over Japan's. China also will not risk military confrontation with the United States to take the islands by force, as the U.S. has unambiguously declared that its security treaty with Japan currently considers the islands Japanese territory. China can continue to press its claims, if only to remind everyone that a dispute exists, but these gestures amount to little more than strategic hot gas—for instance, though China’s ADIZ succeeded in ruffling feathers when it was declared, it has proved largely toothless in practice, leaving China little leverage in future negotiations over the dispute. Furthermore, a view gaining traction in the Chinese foreign policy community is that the islands are not worth the potential harm that animosity with Japan could inflict on China's long-term strategic and economic interests. (Leading Chinese foreign policy scholar Wang Jisi published an article arguing this point shortly before Xi Jinping and Japanese Prime Minister Shinzo Abe's first meeting on the sidelines of last November's APEC summit.) In short, until it can find a way to break Japan's grip over the islands, China's claims in the East China Sea are at a dead end.

Not so, however, in the South China Sea. Chinese control over disputed areas have expanded continuously since the foundation of the People’s Republic; China achieved effective control over the Paracel Islands (nominally also claimed by Vietnam) in 1974 and has grown its presence in the further-flung Spratly Islands since first occupying features there in 1987. The land reclamation efforts allow China to consolidate control in its occupied areas while avoiding direct confrontation with other claimants. And unlike in the East China Sea, where China has little more room to maneuver, China quite literally has plenty of ground left to gain “creating facts on the ground” in the South China Sea. Moving military assets and other infrastructure onto the newly-built frontier islands further serves to strengthen China’s de facto territorial control and also increases China's operational capacity to impose and enforce its claims in the region, giving China considerable leverage in any future negotiations over their status. The balance of military power is also switched in China's favor; just as China would be loath to challenge U.S. firepower to take the Senkaku/Diaoyu Islands, none of the claimant states in the South China Sea—nor the United States—are willing to go toe-to-toe with the People's Liberation Army to dislodge China from the Spratly and Paracel Islands. And if military force can’t impede China’s claims in the South China Sea, then international arbitration—such as the case brought by the Philippines before the Permanent Court of Arbitration in The Hague—hardly stands a chance.

As long as China has more to gain from escalation in the South China Sea, there is unlikely to be a calming of tensions as seen in the East China Sea. Unless the U.S. and its allies in the region can quickly bring China to the negotiating table, Chinese control over many of the South China Sea islands will slowly but steadily become a foregone conclusion. In that case, the U.S. and its allies may need to be prepared to cede a significant amount of territory to China in an eventual deal to prevent China from fully operationalizing its "nine-dash line" claim, which would lasso almost the entire South China Sea into China's territorial waters and potentially threaten freedom of navigation and other countries' resource exploration rights. The clock is ticking in the South China Sea, and each passing minute gives China more reason to keep pressing.

This article is also published on China-U.S. Focus and The Diplomat.