Nothing highlights the growing defense ties between China and Russia more than the fact that Beijing is, for the first time, contributing significant forces to a large Russian military exercise on Russian soil.

Vostok-2018, which is scheduled for Sept. 11-17, is the biggest Russian military exercise in over three decades, involving nearly 300, 000 soldiers, 1,000 aircraft and 900 tanks.

Joining them will be 3,200 troops from the People's Liberation Army -- contributing to a regular Russian exercise that, when held in 2010, was designed to simulate a conflict with China.

Russian Defense Minister Sergei Shoigu has declared that Vostok-2018 "will be unprecedented in terms of geographic scope and the strength of command and control centers and forces due to participate."

Doubtless, western military planners will be watching closely. However, while the West may have underestimated the renewed strength of the Moscow-Beijing relationship, the U.S. and its allies must be cautious about the steps they take in seeking to break it up.

Vostok-2018 adds to an expanding Russia-China security relationship that already includes regular naval exercises, as well as anti-terror drills and the start of antimissile defense cooperation. Officials also describe political ties as being at "the best level in history," and Chinese President Xi Jinping has spoken of his Russian counterpart President Vladimir Putin as "my best, most intimate friend."

Furthermore, bilateral trade grew by more than 30% during the first half of 2018 and may exceed $100bn this year. These economic ties will receive an added boost when Xi makes his first visit to Russia's Eastern Economic Forum in Vladivostok, Sept. 11-13.

The post-Cold War rapprochement between these authoritarian giants has periodically caused nervousness in the West. However, these concerns have been assuaged by the argument that this is an "axis of convenience," in which superficially close relations conceal fundamental problems.

In particular, it has been commonplace to point to historical tensions, including Russia's annexation of Chinese lands in 1858-60 and the Sino-Soviet border clashes of 1969. Commentators have also pointed to the likelihood of friction over Central Asia, where Russia's traditional dominance is being challenged by China's economic penetration. Additionally, there is the issue of Russia's expected unwillingness to accept the junior role within the partnership that its reduced status implies.

There is some merit to each of these points, yet those who were skeptical about the relationship's prospects overlooked the extent to which tensions with the West would motivate Russia and China to set aside their differences. Above all, the West's policy of isolating Russia after the 2014 annexation of Crimea, as well as its intensifying sanctions regime, has forced Moscow ever more tightly into Beijing's embrace. This was most noticeable in the 30-year deal signed in May 2014 for China to buy gas from Russia's Far East, as well as in Russia's agreement in November 2015 to sell China the Su-35, Russia's most advanced fighter jet that it had previously resisted selling to Beijing for fear of the technology being stolen.

Observers also failed to anticipate the skill with which China and Russia would manage their relationship. In particular, Beijing has been mindful of Russian sensitivities in Central Asia and has conducted much of its relations with the region through the Shanghai Cooperation Organization, thereby ensuring that Russia, which is also a member, does not feel bypassed.

The result is that the China-Russia strategic partnership must now be accepted as an established feature of international politics. It will not become a full military alliance, not least because Beijing does not wish to become entangled in Russia's regional difficulties or its volatile relations with the U.S. Nonetheless, this quasi-alliance can play a decisive role in the future of Asia, with Russia providing China with the natural resources, diplomatic support, and military technology that it needs to achieve its goal of regional predominance.

What are the U.S., Japan and their liberal allies to do to resist this combined challenge to the rules-based international order?

Some strategists will inevitably argue that the West must drive a wedge between China and Russia by offering inducements to one or the other. This was the thinking that motivated U.S. President Richard Nixon's famous visit to China in 1972. Now, however, the logical target would be Russia since it is the weaker of the two and is less of a long-term threat.

The West could therefore adopt a policy of engagement, offering economic and political incentives in exchange for Moscow distancing itself from Beijing. At the same time, Western governments could surreptitiously encourage bilateral tensions. The aim would be to leave China isolated internationally and with concerns about the security of its 4,000km northern border.

This goal of drawing Russia away from China has been an underlying motivation of Japanese Prime Minister Shinzo Abe's dogged pursuit of closer ties with Moscow since his return to office in 2012. It is conceivable that such thinking may also have informed President Donald Trump's decision to meet Putin in Helsinki in July 2018.

Such realpolitik could prove successful in the short term since Russia would undoubtedly welcome Western sanctions relief, as well as the opportunity to play China and the U.S. off against each other. Nonetheless, the temptation to make a grand bargain with Russia must be resisted.

It must not be forgotten that, during the last five years, the Putin government has been responsible for the invasion of Ukraine, arming forces which are accused of shooting down a civilian airliner, the hacking of the U.S. presidential election, and the use of a nerve agent on the streets of the U.K. This is to say nothing of its treatment of domestic opponents.

Courting such a regime to weaken China might be sold as the best way to preserve the liberal order, yet, in reality, it would be a betrayal of the very principles that the West purports to uphold.

The U.S. and its allies need to recognize the growing strength of the China-Russia relationship and to wake up to its regional implications. At the same time, however, they must demonstrate that their commitments to international law and justice are more than just rhetorical flourishes and that they will not be traded away for short-term strategic gain.

The West should make it clear that the offer of partnership with Moscow will always be available, yet that this will never be the result of a grubby deal. For the Russian leadership, this leaves a clear choice. Unless they abandon their destabilizing behavior and account for past misdeeds, they face a future of tightening sanctions, isolation from the West, and playing second fiddle to Beijing.

James D.J. Brown is an associate professor in political science at Temple University in Tokyo and a specialist on Russo-Japanese relations.