One of the striking warnings in a recent Pentagon white paper on the growing strategic threat from Russia is that its president, Vladimir Putin, could pull a “reverse Nixon” and play his own version of the “China card” with the United States, a reference to the former president’s strategy of playing those two adversaries against each other.

Until recently, any relationship between Russia and China could largely be dismissed as a marriage of convenience with limited impact on American interests. But since Western nations imposed sanctions on Russia after it invaded Ukraine in 2014, Chinese and Russian authorities have increasingly found common cause, disparaging Western-style democracy and offering themselves as alternatives to America’s postwar leadership. Now China and Russia are growing even closer, suggesting a more permanent arrangement that could pose a complex challenge to the United States.

“The world system, and American influence in it, would be completely upended if Moscow and Beijing aligned more closely,” John Arquilla, a professor at the Naval Postgraduate School, wrote in the report, to which Defense Department officials and other analysts contributed.

The latest evidence of warming ties was a meeting last month in Moscow at which Mr. Putin thanked the Chinese leader, Xi Jinping, for enabling the two countries to do more than $100 billion worth of trade in 2018, up 30 percent from the previous year, and an implicit rebuke to America’s trade standoff with China. The two countries also signed more than two dozen agreements . That meeting came shortly after Mr. Xi called Mr. Putin his “best and bosom friend.”