The idea of admitting other Eastern European countries into NATO, however, was still considered recklessly provocative toward Russia. The New York Times editorial board and its star foreign-affairs columnist, Thomas Friedman, strongly opposed the idea. The eminent Cold War historian John Lewis Gaddis wrote that, “[H]istorians—normally so contentious—are in uncharacteristic agreement: with remarkably few exceptions, they see NATO enlargement as ill conceived, ill-timed, and above all ill-suited to the realities of the post-Cold War world.” George H.W. Bush’s national security advisor, Brent Scowcroft, was skeptical of the idea, as was Bill Clinton’s defense secretary, William Perry.

For his part, Russian President Boris Yeltsin warned that extending NATO violated the “spirit of conversations” between Baker and Gorbachev, and would produce a “cold peace” between Russia and the West. It didn’t matter. In 1995, NATO went to war against Serbia, and then sent peacekeepers to Bosnia to enforce the peace agreement that followed. This new, Eastern-European mission paved the way for further expansion. By 1997, it was clear Poland, Hungary, and the Czech Republic would enter the alliance. In 2004, NATO admitted another seven former Soviet bloc countries, three of which—Lithuania, Latvia, and Estonia—had been part of the USSR. In 2009, Croatia and Albania joined the club. Six former Soviet republics—Ukraine, Georgia, Moldova, Kazakhstan, Armenia, and Azerbaijan—now link their militaries to NATO’s via the “Partnership for Peace” program. All five former Soviet republics in Central Asia—Kyrgyzstan, Tajikistan, Turkmenistan, Kazakhstan, and Uzbekistan—provide NATO countries with some basing, transit, refueling, or overflight rights for use in the Afghan war.

From Putin’s perspective, in other words, the United States hardly looks in retreat. To the contrary, the post-Cold War period has brought one long march by America and its allies closer and closer to the border of Russia itself. But there was no reason to believe that Russia—which under Putin has been regaining its confidence on the world stage—would go on contracting forever. And by 2008, when Russia sent troops into parts of Georgia, it was already clear that NATO’s expansion onto former Soviet soil had come to a halt.

It had stopped for the same reason that General Dwight Eisenhower, determined at the end of World War II to keep the American death toll as low as possible, refused to push into Eastern Europe to prevent the USSR from dominating the region after the war. And for the same reason that President Eisenhower watched Soviet troops crush protests in Budapest in 1956, and President Johnson watched Soviet troops crush the Prague Spring in 1968. The frontiers of American power in Eastern Europe have long been set by Moscow’s willingness to send troops into countries where, by virtue of their geography, Russia is prepared to take casualties and the United States is not. (Just as the limits of Soviet power in the Americas were set in 1962 when John F. Kennedy proved more willing to risk war over Soviet missiles in Cuba than did Nikita Khrushchev.)