A warming friendship with China is allowing Russia to turn up the heat on Ukraine

• Russia's massive military buildup around Ukraine includes thousands of troops from its Eastern Military District.
• Those troops are responsible for Russia's border with China, and their movement illustrates Moscow's comfort with its neighbor.

Russia and Belarus on Thursday kicked off a joint military exercise involving 30,000 of the 130,000 troops that Russia has massed on its border with Ukraine in recent months, again raising concerns about a Russian attack on Ukraine.

Moscow has repeatedly denied it plans to attack its neighbor, but it has continued to gather troops nearby — including roughly two-thirds of the combat forces assigned to its Eastern Military District, which is responsible for the far eastern regions some 6,200 miles away.

The shift of so many troops so far west has been enabled by Russia's increasingly warm relationship with its largest neighbor and one-time rival, China.

The Eastern Military District forces that are now in central and western Russia and southern Belarus are a mix of motorized infantry, armor and artillery, and airborne troops, said Konrad Muzyka, director of Rochan Consulting, a Poland-based military analysis and consulting firm.

Some of those Eastern Military District forces, including S-400 and Pantsir-S air-defense systems, Su-35 fighter jets, and Su-25SM attack aircraft, are participating in the exercise with Belarus, according to Russian news agency Interfax.

The drills are to last 10 days, but it's not clear how long troops from the Eastern Military District can remain this far from their home bases.

"I would probably say this is likely a short-term deployment more broadly — a few weeks, a couple of months," Muzyka told Insider on Wednesday, calling that "an educated guess."

Russia has moved troops and equipment to its western regions before, but the number deployed now and the locations to which they are deployed is unprecedented, and recent moves to link troops with pre-positioned equipment suggest a new phase in their deployment is beginning, Muzyka told Insider.

"Russia has never done anything like this before," Muzyka said.

Comfortable with vulnerability

The number of Eastern Military District forces that have moved to Belarus "is illustrative of the degree of vulnerability Moscow is comfortable with" on its border with China now, according to Michael Kofman, research director in the Russia Studies Program at the research organization CNA.

"The Russian Far East has probably not been stripped of combat power like this in decades," Kofman said last week.

Moscow and Beijing were partners early in the Cold War, but their relations frayed in the 1960s, and they briefly fought over a disputed part of their border in 1969.

Moscow and Beijing normalized relations in 1989 and improved ties throughout the 1990s. They signed a friendship and cooperation treaty in 2001 (which was renewed last year) and in 2008 they signed a pact settling their 2,700-mile border.

The buildup in Belarus and around Ukraine "is partly possible because of the border treaty with China," Alexander Gabuev, chair of the Russia in the Asia-Pacific Program at the Carnegie Moscow Center, said last week.

The number of Russian troops on the border with China and Mongolia now "is at a historic low since 1922," which is "the byproduct of the Sino-Russian relationship being improved," Gabuev said.

Artyom Lukin, a scholar of international relations at Russia's Far Eastern Federal University, told Insider it was "fair to say that Russia's land border with China is relatively unprotected now" and that it "could be a good time" to invade if China wanted Russian territory.

But China is focused on Taiwan and the South China Sea and on preparing for a potential war with the US. "This is one major reason why China needs good relations with Russia," Lukin said. "Moscow knows this, which explains the Kremlin's lack of concern with leaving Russia's eastern border exposed."

'Never against each other'

Russia and China's relationship has advanced on many fronts, especially after Russia's 2014 annexation of Crimea, which prompted Western backlash that further isolated Russia. Their progress in the military realm has perhaps been the most visible.

China has for decades relied on Russia for military hardware and education. China's participation in Russia's Vostok exercise in 2018 was a milestone in their military-to-military exchanges. They conducted another joint exercise in China last year where their troops used a joint command-and-control system for the first time.

Their militaries have also conducted joint bomber patrols over disputed areas of the Pacific in 2019, 2020, and 2021, and in November their navies did a first-of-its-kind joint patrol around Japan, which was seen as a message the region.

They don't have a formal alliance, but a statement issued after a summit between leaders Vladimir Putin and Xi Jinping this month showed both sides pledging to stand together more closely against the West and was what one expert called an "inflection point."

A post-summit statement by the Kremlin described significant steps toward each other's positions on key issues, saying both sides "oppose further enlargement of NATO" and were "seriously concerned" about the recent security pact between the US, UK, and Australia.

There are still potential fault lines. Experts point to possible disputes over influence in their border regions, Central Asia, and the Arctic, as well as to differences arising from China's global ascendancy and Russia's relative decline.

They also differ on important territorial issues. The post-summit statement included Russia's reaffirmation of support for Beijing's "One-China principle" regarding its sovereignty over Taiwan. China, however, hasn't recognized Russia's annexation of Crimea.

Beijing may look coolly on another Russian invasion of Ukraine, which could have economic and political consequences for Chinese leaders. Kevin Rudd, a former Australian prime minister, noted Thursday that China's English-language post-summit statement was one-quarter the length of Russia's and "excludes all the bits relevant to Europe and NATO."

Russia and China are likely to continue working hard to tamp down any potential disputes and to maintain what Kofman has called "a functional non-aggression pact" that allows them to focus on their disputes with the US.

"'Not always with each other, but never against each other' is the formula on security and on the border," Gabuev said of Sino-Russian relations.

While US experts have advocated efforts to exploit areas of disagreement between Russia and China to break up their partnership, there are doubts that they can be coaxed into conflict or that US policy could shift enough to do so.

Frustration with US foreign policy "trumps whatever ambivalence — historically, ideologically, interpersonally, structurally — might exist" between Moscow and Beijing, Evan Feigenbaum, vice president for studies at the Carnegie Endowment for International Peace, said last week.

"I don't see anyone in Washington that's willing to consider a wholesale reevaluation of American policy toward China, toward Russia, or toward both of them," Feigenbaum said.