If you want to understand why President Vladimir Putin is escalating today’s Russian-Ukrainian tension to block Kyiv from gaining NATO membership and infrastructure, Mary Sarotte’s new book provides important context. It explains the crucial importance of past NATO expansion to Russian relations with the United States and its allies today. I asked Sarotte, who is the Kravis professor at the Johns Hopkins University School of Advanced International Studies, to explain the relevance of her book,Not One Inch: America, Russia, and the Making of Post-Cold War Stalemate,” which just appeared in hardback.

HF: Putin recently claimed that NATO expansion had undermined Russian security. What does your book tell us about the history behind this claim?

MS: Putin complained, in a speech on November 18, that the “military infrastructure of the NATO bloc” has been “deployed right next to our borders.” He called for “serious long-term guarantees that ensure Russia’s security,” given that Westerners are “not very reliable partners [and] can easily backtrack on any previous agreement.”

In “Not One Inch,” I show how the origins of his complaint go back 30 years, to the decade after the unexpected demise of the Soviet Union in December 1991. Using formerly secret records of White House-Kremlin contacts, many of which I got declassified, I investigate how the United States overcame Russian resistance after the Soviet collapse and began enlarging NATO to a billion people.

New member states were thrilled to join, but there were unintended consequences: Washington’s hardball tactics, combined with Moscow’s self-inflicted wounds, set both countries on the path to today’s renewed hostility, just as Putin was rising through the ranks in Moscow.

The heart of the story is how, during the 1990s, the meaning of “not one inch” shifted dramatically. These words had originally formed a hypothetical exchange, proposed in February 1990 by the U.S. secretary of state, James Baker, to Moscow: Roughly, you let your half of divided Germany go — so the country can unite after the November 1989 fall of the Berlin Wall — and we move the Atlantic alliance not one inch eastward.

After the Soviet collapse in 1991, however, Washington realized that it could not only win big, but win bigger. Not one inch of territory needed to be off-limits to full NATO membership.

My book traces how that fateful shift in meaning happened, and how it sidelined a previous, smarter strategy for NATO enlargement: one including an affiliation for Ukraine that was, remarkably, acceptable to Moscow.

“Not One Inch” illuminates not just the American but also the Russian agency in this story. Although there is no written agreement preventing NATO expansion eastward across the former Cold War line — and what survives in writing expressly permits the alliance to cross that line — Putin has nonetheless repurposed this history to justify his actions. His current threats to Ukraine make this controversy even more explosive.

HF: You quote a Soviet politician as saying that Mikhail Gorbachev, the last leader of the Soviet Union, “will remain in history as a messiah” but was “lost as a politician.” Would NATO’s expansion have gone differently if the Soviet Union had a cannier leader?

MS: Possibly, but there is always more than one cause for major events. Changing only one factor, namely Gorbachev’s level of negotiation savvy, would not clearly have caused another outcome. But although his idealism had a far-reaching impact — if forced to identify one person who pulled the world back from the Cold War thermonuclear standoff, I would have to credit Gorbachev — the evidence does strongly suggest that his negotiation skills were weak and that the West outmaneuvered him.

Gorbachev also had determined domestic opponents, including his nemesis, Boris Yeltsin, who was perfectly willing to dismantle the Soviet Union to undermine Gorbachev. As former U.S. national security adviser Brent Scowcroft concluded, “the Soviet Union was disintegrating, I think, almost completely because it was the way Yeltsin could get rid of Gorbachev.”

HF: Bill Clinton’s marital infidelity had big geopolitical consequences. What were they?

MS: In my research, I came across one of Clinton’s advisers noting that the president seemed to be “sleepwalking” through a particular summit. That led me to read the testimony of Monica Lewinsky, the White House intern whose relationship with Clinton prompted his impeachment, and to put together a timeline of their interactions.

It turned out that those interactions overlapped heavily with milestones in NATO enlargement, including the summit at which he was sleepwalking. One time Clinton fought with Lewinsky — because she hinted that she would go public — immediately before boarding his flight for the 1997 Madrid summit that started NATO expansion beyond Germany.

To be sure, Lewinsky did not make NATO policy. But Clinton’s relationship with her had consequences for NATO, and indeed for all of his policymaking. Fear of discovery distracted him; public revelation limited his political room to maneuver; and impeachment meant that consultations with personal lawyers increasingly edged out sessions with foreign policy advisers, to the despair of Secretary of State Madeleine Albright and others.

HF: Despite continuities across different U.S. administrations, there were ferocious internal arguments over whether and how far to expand NATO. How might things have turned out if the skeptics of expansion had won their internal battles?

MS: “Not One Inch” widens the aperture beyond the usual binary debate over whether NATO expansion was either good or bad. Instead, the book shows that there were multiple possible ways to enlarge the alliance proposed at the time. These alternate strategies could have achieved similar results at a lower “cost per inch” of expansion, as measured in units of damage to Western relations with Moscow. Put differently, NATO enlargement was a reasonable policy; problems arose when the United States decided on an all-or-nothing approach to expansion that maximized conflict with Moscow.

Now, Putin’s current threats show that we are back on the brink. Putin apparently cares about marking birthdays and anniversaries, and now he apparently wants to mark the 30th anniversary of the Soviet demise by ending NATO expansion for good.