In his recent commentary, “The Avoidable Russia-China Romance,” Nikolas Gvosdev provides a strong case that despite recent examples of teamwork between the two powers, a sustained collaboration is hardly inevitable. Gvosdev focuses on the ways in which the United States can limit the risk that a “Eurasian entente” will arise in the near term, refuting the notion that such an alignment is historically determined.

Regarding the Sino-Russian relationship through a wider temporal lens not only reinforces Gvosdev’s conclusion, but suggests that the tide of history may begin to drag the two nations towards contention rather than conspiracy. Recent instances of tactical and diplomatic cooperation between Beijing and Moscow show why Washington should remain open to working with each, yet these initiatives also hide an obstacle in the path to partnership. Set when Russia acquired a portion of Manchuria in the nineteenth century, a demographic time-bomb may bring any marriage of convenience to an unhappy end.

In the grand scope of Chinese history, Russia’s presence in Northeast Asia is a recent development. Until the reign of Ivan III, which ended in 1505, Muscovy held less than 3 percent of present-day Russia, and it was not until 1639, at the end of the European Age of Discovery, that Russian explorers would first reach the Sea of Okhotsk. Russia arrived in the region in an era of tumult, as the sinking Ming Dynasty tried to contain a peasant rebellion and stave off waves of incursions by Manchu horsemen, who finally captured Beijing in 1644, establishing the dynasty that would become known as the Qing.

During the late seventeenth century, Cossacks would clash with Manchu forces along the Amur River, which today separates the Russian Far East from the northeastern Chinese provinces of Heilongjiang and Jilin. The Manchus mostly got the best of the interlopers, and the first Sino-Russian treaty, signed in 1689, awarded the Qing territory north of the Amur in exchange for Russian traders’ access to Chinese markets. Henceforth, Moscow would adopt a canny strategy of free-riding, slowly advancing when the Qing faced other external or internal threats, but it would need to wait nearly two centuries, until China’s final imperial dynasty had entered its death spiral, to capture all of coastal Manchuria.

In contemporary China, it is known as the “century of humiliation”: the period starting with Britain’s victory in the First Opium War in 1842 and lasting at least until Mao Zedong’s declaration of the People’s Republic in 1949. Beginning in the 1850s, Russians seized advantage of Chinese disarray to pressure the Qing’s northern frontier, from restive Muslim Xinjiang in the west to the Amur River in the east. Two “unequal treaties” in 1858 and 1860 gave Russia more land than it had conceded to the Manchus two hundred years earlier, including the vast region then called Outer Manchuria. The southern part of this concession would become Russia’s maritime province, Primorsky Krai, where the city of Vladivostok (“Ruler of the East”) was chartered in 1880.

The century that followed Moscow’s “Amur annexation” introduced dramatic changes to Northeast Asia’s political landscape, but did not erase Chinese resentment over this lost territory, which was to become a significant irritant during the Sino-Soviet split. What started as an ideological dispute between the world’s two largest communist states led to a set of border clashes, the most explosive of which occurred along an Amur River tributary in 1969. Five years earlier, Mao had infuriated Moscow when he told visiting Japanese reporters that much of the Russian Far East was stolen land; now, he instructed Chinese premier Zhou Enlai to press this inflammatory line in emergency talks with his Soviet counterpart. The risk of war passed despite Beijing’s provocative rhetoric, but Chinese bitterness did not—as Henry Kissinger would learn during his first meeting with Deng Xiaoping in 1974, when the topic of a U.S.-Soviet arms summit in Vladivostok prompted the future Chinese leader to lecture him on Russian rapacity.

Considering this background, as well as China’s unresolved boundary disputes with other neighbors, it is remarkable that Beijing and Moscow have since been able to settle all their territorial differences. Although the decisive round of border talks started before the formal disintegration of the Soviet Union, the Kremlin’s unfamiliar weakness vis-a-vis Beijing helped to shape their outcome. Some western spats were removed from the table by the independence of the Central Asian states, but disputes over small pieces of land along the Manchurian frontier were generally resolved in China’s favor. Beijing, for its part, did not contest the basic validity of the “unequal treaties” of 1858 and 1860, leaving the Russian Far East untainted by controversy.

Such, at any rate, goes the official line. For although China currently maintains no claims to Russian land, many in Moscow remain convinced that Beijing has not given up on the Far East forever. Fueling Russian fear is a fantastic population imbalance and a wave of illegal Chinese immigration which could eventually render European Russians a regional minority. With 110 million residents—and 65 million in the border provinces of Heilongjiang and Jilin alone—the northeast holds only 8 percent China’s population but is more than three quarters the size of Russia’s, which is heavily concentrated west of the Ural Mountains. With around 6 million people, the Russian Far East is among the most vacant places on earth and is only growing emptier, as nationwide demographic collapse is compounded by out-migration. Endowed with oil, gas, coal and timber, the region is the opposite of nearby China: rich in resources while starved for labor and capital.

Thus, although Moscow and Beijing recently staged their largest-ever joint naval drill off the coast of Primorsky Krai, Russia has continued to run exercises which appear to be aimed at China—including a 2010 ground drill tailored to repel an invasion by an unnamed foe resembling the People’s Liberation Army, and massive war games held just last month. In addition, the Kremlin has maintained its time-honored partnership with India, and has also sought to improve ties with China’s archnemesis Japan, pledging to negotiate a long-delayed World War II peace treaty, which would not only sow the seeds for additional Japanese investment in Far East oil and gas fields, but could provide a hedge against Chinese economic and military coercion.

The idea that Beijing might march on Vladivostok is obviously far fetched, but it is not terribly hard imagine well-placed hawks musing about the legitimacy of Russia’s borders if the two powers should find themselves at odds, just as Mao and Zhou did in the 1960s. (In a more recent example of such irredentist escalation, scholars at a state-backed Chinese think tank questioned Japan’s claim to Okinawa.) Indeed, far beyond Manchuria, possible sparks already abound: from Central Asia, where booming trade with China threatens to erode Moscow’s traditional sphere of influence, to the South China Sea, where Russia has worked closely with Vietnam to develop and safeguard energy resources claimed by Beijing. “In the parts of the world that matter to them most,” Jeffrey Mankoff has noted, “Russia and China are more rivals than allies.”

The fact that it would be self-defeating for Beijing to undermine one of its greatest diplomatic achievements of the last several decades by resurrecting the treaties of 1858 and 1860 is one reason to think the issue will remain interred. At the same time, however, China has exhibited a perplexing tendency to poke its neighbors in the eyes, a strategy some experts have dubbed “self-containment.” Even since 2010, when Beijing’s aggressive posture in its near abroad produced a significant diplomatic backlash, China has continued to press and provoke. This year alone, Beijing has heightened the stakes in a dangerous standoff with Tokyo over the Senkaku Islands, and Chinese patrols have repeatedly strayed across the line of control in areas disputed with India. While Chinese nationalists loathed the Manchu Qing in the early twentieth century, they are now inclined to embrace its capacious borders—and if Beijing’s hawks grow louder in an era of slowing growth, some may recall the Russian indignities that Mao and Deng never forgot.

Needless to say, none of this promises an immediate end to Russo-Chinese collusion, nor does it obviate the need for Washington to pursue pragmatic policies designed to avert such a partnership. Vladimir Putin and Xi Jinping will continue to seek common ground, if only as a result of their shared discomfort in an international system built and maintained by the United States and its allies. As Leslie Gelb and The National Interest publisher Dimitri Simes noted in The New York Times last month, both nations “appear to have decided that, to better advance their own interests, they need to knock Washington down a peg or two.” Especially on the UN Security Council, the two authoritarian powers will continue taking positions that reflect their interest in curtailing American influence and thwarting what they each view as meddling in sovereign nations’ internal affairs.

On the other hand, relentless demographic trends in Northeast Asia suggest that any collaboration between Moscow and Beijing will operate under a cloud, which could grow darker as China’s relative military strength increases. Even if Chinese leaders try to reassure Moscow that its hold on the Far East is secure, both states surely know that the growth of the region’s Chinese population amidst Russian decline may place the other in a bind, with nationalist pressure setting constraints on compromise. While Putin and Xi grip and grin, the demographic time-bomb between them is ticking—and if it goes off, a shared suspicion of the United States may prove a brittle bond.

Taylor Washburn is a lawyer studying at the Johns Hopkins School of Advanced International Studies, and was previously a visiting professor at the Korea Advanced Institute of Science and Technology. He can be followed on Twitter @washburnt.