A famous Marine Corps general once said to me — and to most of the Pentagon brass — that the key thing we need to do, as a nation, is prevent Russia and China from getting closer. At the time the prospects of a Chinese-Russian alliance seemed far fetched and the goal of keeping them apart eminently doable. 

Four years later, an alliance not only doesn’t seem ludicrous, it seems perfectly likely. The two nations are closer than they have been since their Cold War alliance, which later collapsed. Their leaders — Xi Jinping and Vladimir Putin — have met more than 40 times in 10 years. 

In February, Presidents Xi and Putin announced that their relationship had “no limits.” China is Russia’s largest trading partner after the EU, with energy comprising roughly half of Russia’s exports to China. Messrs. Xi and Putin recently pledged to increase trade by 50 percent by 2024.

The duo also plan to build new energy pipelines connecting the two countries — in addition to Russia agreeing to protect China’s overland pipelines to Russia in the event of a conflict, a move that would dramatically undercut any blockade or sanctions regime the United States might attempt to put in place.

Russia and China are also increasingly coordinating their votes at the United Nations and moving to “de-dollarize” their trade to limit exposure to the West, its banking system, and sanctions. Russia has also sold China tens of billions of dollars’ worth of weapons.

The weapons sales have included air defense systems, combat aircraft, helicopters, and submarine capabilities, with a sharp increase in transfers taking place since 2015. Russia and China are also collaborating on new weapons development, to include missile warning systems and space operations.

According to a leading China-watcher at Stanford University and the American Enterprise Institute, Oriana Skylar Mastro, we are seeing evidence of integrated supply chains, with Russian factories increasingly manufacturing Chinese military equipment. The Russian and Chinese defense industries, she says, are becoming increasingly mutually supportive.

The Russians, Dr. Mastro says, could provide the Chinese with material especially vital for war-fighting, such as petroleum oil and lubricants. This would be a part of a broader effort, she argues, for the Russians to act as a strategic rear for the Chinese in the event of war in the Pacific, most likely over Taiwan.

In addition, Dr. Mastro reckons, the Russians could hold down certain areas or territory to allow the Chinese to focus their efforts elsewhere. Russia has set up certain defense and air identification zones that would deny America the ability to fly in vital areas, including around Japan.

Military cooperation between the two nations includes growing interoperability. With growing frequency, Russia and China are exercising together both on land and at sea. They have emerged as each other’s key training partner. According to Dr. Mastro, many of these drills involve Russia helping China hone tactics for a fight against America. 

What the Chinese stand to get from this arrangement is clear. Whether the relationship is reciprocal is another question. While China has refrained from criticizing Russia’s invasion of Ukraine, and provided an economic lifeline to Russia amid Western sanctions, China does not appear to have been giving much in the way of military assistance to Russia. When questioned about the depth of the Russian alliance, according to Dr. Mastro, Chinese officials make it clear that they believe the scope is limited to coordination in Asia. They claim to have no intention of getting involved in Europe.

It might be that the best thing Russia feels it gets from helping the Chinese militarily is an opportunity to weaken America, itself a longstanding goal of Moscow. Russia may not be strong enough to challenge the United States directly, but if it can weaken us through China, Moscow would consider that a win. 

So far, the China-Russia alliance is deep but informal. That may be intentional, for flexibility and strategic ambiguity. Dr. Mastro calls it “an alliance gray zone” that dilutes would-be opposition. 

Take the case of Japan and South Korea. At the moment, they have little interest in fighting China to defend Taiwan. It would probably just be American alone. If China and Russia were to form an official military alliance, though, that might change. And if there is one goal the Chinese have, it is to prevent a unified coalition from forming against them in the Pacific. 

It seems their “gray zone alliance” strategy is working. By all accounts, the United States isn’t doing much to prevent — or prepare for — this growing alliance between our two greatest adversaries. That is a serious mistake. A successful and deepening Russia-China alliance would severely undercut American leverage and inhibit our ability to pursue our goals and defend our interests on the world stage. 

Together, these economies wield significant economic might (China) and energy (Russia). Both nations have large territories, major populations, and global ambitions. Both have invested heavily in their militaries and next-generation technologies. Both possess large and skillful cyber capabilities. The vast majority of malicious cyber activity today can be tied back to one of those two countries. They also control two-fifths of the United Nations Security Council, meaning a double veto on any international agreements at the world body. 

Needless to say, if these two blocs come together with real cooperation and an investment in each other’s success to begin operating in concert, the United States would be in trouble. Consider us blocked. 

First of all, an alliance between them largely undercuts the viability of our sanctions strategy, a tool we use regularly to penalize behavior we don’t like without resorting to military force. We have already seen this play out to undercut the effectiveness of Western sanctions over Ukraine. 

Second, in the event of a war, the best strategy we have to limit scope, minimize casualties, and bring hostilities to a rapid close is to isolate our adversary diplomatically, economically, and militarily. This is hard enough to do with nations like Russia and China in normal circumstances. It would be nearly impossible to do in the event of an alliance. The United States is unquestionably the best military in the world, but we would, nonetheless, be hard-pressed to fight Russia and China at the same time. And which of our allies, in NATO or otherwise, truly has the hard combat power to make up the difference?

Not that all is lost. Collaboration is difficult. It’s one thing for two countries to say they’re going to fight together; it’s another to actually do it. We still struggle with NATO after decades together to solve language challenges, ensure that systems work, develop a common doctrine, and train.

Nor do the Chinese and Russians make natural allies. Their ideologies don’t match. They have long had disputes along an extensive border. Their interests diverge. Russia isn’t worried about the Pacific, and China isn’t interested in getting involved in Europe. Distrust runs deep on both sides. They would each be threatened by the other growing too powerful. It may be that the thing they have most in common is being pariahs to the Western world. 

All this is a bit too hopeful for me. Maybe, like in the past, the relationship will decompose on its own. Hal Brand of Johns Hopkins University has reportedly even suggested that the best thing we can do is just push the two countries closer on a bet that, as in the Cold War, they would eventually disappoint each other.

Potentially, that is already happening. The Russians allegedly asked the Chinese for equipment or troops to fight in Ukraine, and the Chinese denied them. Perhaps that refusal will shift Mr. Putin’s willingness to stick his neck out for Mr. Xi in the Battle for Taiwan, or any other issue. But that posture could also be changing. Last week, the American commerce department blacklisted five Chinese companies for helping Russia’s military. 

Only time will tell. What, though, is the logic of leaving such a potentially existential  matter to fate? Nations need allies; this wouldn’t be the first time the enemy of my enemy became a friend. Given the stakes, our leadership needs to wake up, heed the advice of the sage general, and put the issue front and center where it belongs.