Rather than a sign of aggression, the fact that the two nations can modernize their militaries and upgrade missiles in what was once a highly contentious area underscores how close they have become since the end of the Cold War. This newfound trust spells trouble for the United States and its partners in the region as Beijing and Moscow share an aversion to American leadership in the Asia-Pacific.

The frozen tundra along Russia’s far-eastern border with China is becoming a hot zone as both nations deploy nuclear-capable missiles to the area. Are two of the world’s most advanced militaries preparing for war with one another?

In June, Russia armed a fourth brigade in the far east with the deadly Iskander-M short-range ballistic missile system. There are now twice as many brigades equipped with these nuclear-capable missiles on the Chinese border than any other Russian military district. The Iskander-M has a range of 250–310 miles, which puts China squarely in its sights.

(This first appeared last August.)

South of the border, China has reportedly moved the Dongfeng-41, an intercontinental ballistic missile capable of carrying multiple nuclear warheads, to its northeastern-most province.

Though the two nations appear to be close allies, conducting joint military exercises in recent years, this region has been a significant source of tension in the past. In 1969, the Soviet Union and China nearly went to war with one another following deadly military clashes in the area.

By deploying Iskander-M missile brigades, some analysts suggest that Russia is seeking to check the increasing military and economic might of its southern neighbor.

Perched along Russia’s eastern-most border are over 100 million Chinese to just 4 million Russians. Based on some estimates, as many as five million Chinese migrants have poured across the border, sparking fears that they could become the region’s dominant ethnic group. Vladimir Putin has even warned residents that their children could one day grow up speaking Chinese.

Many of the areas surrounding the border once belonged to the Middle Kingdom, and the area is particularly attractive to the rapidly growing nation’s insatiable factories. The region contains almost all of Russia’s diamonds, a third of its gold, and major oil, natural gas and zinc deposits.

Although both sides have historic and economic interests in the area, and the military buildup could be interpreted as an aggressive power play, there is no risk of war. China vehemently denies deploying an ICBM to its northern border, and Russia insists that the two nations remain close allies. Furthermore, given the Chinese Dongfeng-41’s range and its flight trajectory, moving it closer to Russia would actually limit the areas within the country that it could target.

So why then are these two countries militarizing this sensitive border?

Far from preparing for war with one another, the military buildup in the region is the legacy of Sino-Russo tensions, Cold War military strategy and a lack of funding to build new Russian military infrastructure.

At the height of the Sino-Soviet split in the late 1960s, China feared a simultaneous Russian assault through Mongolia into Beijing and a multi-pronged invasion through northeastern China. Outmatched by Russia’s military at the time, China opted to use the country’s size and its overwhelming numerical advantage to halt such an attack. This resulted in constructing defensive positions and stationing millions of troops south of the border.

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The Soviets worried that they would not be able to withstand a massive Chinese incursion, which would threaten strategic positions in the east and crucial points along the Trans-Siberian Railroad. Senior Soviet defense officials took the threat so seriously that some even considered deploying nuclear mines along the border or conducting a pre-emptive nuclear strike.

Decades have passed and the threat of war has long subsided, but the military infrastructure for such an event remains in place.

According to Peter Wood, an Asian security researcher, the concentration of Chinese troops aimed at preventing a Russian invasion from the north has not changed much since 1973. However, in a sign that China no longer perceives Russia as a threat, the military command responsible for defending this border was downgraded from the highest priority to the second lowest as part of a broader military reorganization in 2015.

The circumstances are similar on the Russian side of the border.

“The geographic deployment of Russia's Armed Forces is closely interconnected with historic circumstances of the collapse of the Soviet Union,” said Russian military analyst Vasily Kashin.

During the Cold War, the Soviets stationed their most battle-ready brigades in East Germany and in Eastern Europe. But after the collapse of the Soviet Union, the most powerful military units left in Russia were those stationed in the far east for a war with China.

Russia lacks the resources to build new military infrastructure like barracks, airfields and warehouses, so it must rely on Soviet-era bases, explained Kashin. As a result, Iskander-M missile brigades are now stationed at the positions where China had once anticipated a Russian invasion with one brigade in Mongolia, another brigade near Inner Mongolia, and two along China’s northeastern-most border.

Rather than a sign of aggression, the fact that the two nations can modernize their militaries and upgrade missiles in what was once a highly contentious area underscores how close they have become since the end of the Cold War. This newfound trust spells trouble for the United States and its partners in the region as Beijing and Moscow share an aversion to American leadership in the Asia-Pacific.

Eugene K. Chow writes on foreign policy and military affairs. His work has been published in The Week, Huffington Post, and The Diplomat.