So it went for almost half a century, until the Soviet empire collapsed. Immediately, some international-relations scholars predicted a return to old-fashioned great-power rivalry. In 1990, the University of Chicago’s John Mearsheimer published an essay entitled “Back to the Future,” in which he predicted a new “multipolar” competition resembling the one that held sway in the 19th century. This competition, Mearsheimer predicted, would be less ideological than the Cold War, but more unstable, and might plunge Europe into war.

It didn’t happen. To the contrary, NATO—having won the Cold War—expanded, and no adversary rose to challenge it. This absence of great-power strife enabled the massive exchange of money, people, culture, and ideas dubbed “globalization.” Even after 9/11, the era of relative great-power harmony endured as the world’s strongest countries largely cooperated against terrorism. “We have an historic opportunity to break the destructive pattern of great power rivalry that has bedeviled the world since [the] rise of the nation state in the 17th century,” declared Secretary of State Condoleezza Rice in 2002. “Today, the world’s great centers of power are united by common interests, common dangers, and—increasingly—common values.”

Although Americans didn’t think much about it at the time, this absence of great-power tension enabled much of what the United States did in the war on terror. Had the Soviet Union not withdrawn its troops from Afghanistan and orphaned its former client, Iraq, the U.S. could never have invaded and occupied those countries. Had China seriously challenged American power in the Pacific, the U.S. would never have enjoyed the luxury of focusing its attention and resources so overwhelmingly on the greater Middle East. Terror networks like al-Qaeda and small “rogue” states like Iraq dominated American consciousness because big powers like Russia and China stood largely offstage.

That’s what’s now changed. The risk of jihadi terror remains; Iran is still seeking the capacity to build a nuclear bomb. But these threats appear comparatively smaller when Russia occupies Ukraine or, as happened last November, China erects an air-defense zone over most of the East China Sea. Just look at how Putin’s actions have pushed Israeli Prime Minister Benjamin Netanyahu’s Iran-focused visit to Washington this week off the front page.

When there’s serious tension between America and other major powers, that tension becomes the dominant reality in U.S. foreign policy. And it’s likely that tension will endure. Vladimir Putin has now twice invaded his neighbors in an effort to halt, if not reverse, the West’s encroachment into the former U.S.S.R. Yet the more bullying he becomes, the more desperately many in Ukraine, Georgia, Moldova, and perhaps other ex-Soviet republics will seek economic and military bonds with Europe and the U.S. Large chunks of the former Soviet Union now constitute a gray zone where competition between Russia and the West can breed diplomatic feuds, economic sanctions, and even proxy war.