Leslie H. Gelb, a former columnist, editor and correspondent for The New York Times, is president emeritus of the Council on Foreign Relations. Dimitri K. Simes is president of the Center for the National Interest and publisher of its magazine, The National Interest.

THE flight of the leaker Edward J. Snowden from Hong Kong to Moscow last month would not have been possible without the cooperation of Russia and China. The two countries’ behavior in the Snowden affair demonstrates their growing assertiveness and their willingness to take action at America’s expense.

Beyond their protection of Mr. Snowden, Chinese-Russian policies toward Syria have paralyzed the United Nations Security Council for two years, preventing joint international action. Chinese hacking of American companies and Russia’s cyberattacks against its neighbors have also caused concern in Washington. While Moscow and Beijing have generally supported international efforts to end Iran’s nuclear weapons program, they clearly were not prepared to go as far as Washington was, and any coordinated shift in their approach could instantly gut America’s policy on the issue and endanger its security and energy interests. To punctuate the new potential for cooperation, China is now carrying out its largest ever joint naval exercises — with Russia.

Russia and China appear to have decided that, to better advance their own interests, they need to knock Washington down a peg or two. Neither probably wants to kick off a new cold war, let alone hot conflicts, and their actions in the case of Mr. Snowden show it. China allowed him into Hong Kong, but gently nudged his departure, while Russia, after some provocative rhetoric, seems to have now softened its tone....