Newly available sources show how the 1993–95 debate over the best means of expanding the North Atlantic Treaty Organization unfolded inside the Clinton administration. This evidence comes from documents recently declassified by the Clinton Presidential Library, the Defense Department, and the State Department because of appeals by the author. As President Bill Clinton repeatedly remarked, the two key questions about enlargement were when and how. The sources make apparent that, during a critical decisionmaking period twenty-five years ago, supporters of a relatively swift conferral of full membership to a narrow range of countries outmaneuvered proponents of a slower, phased conferral of limited membership to a wide range of states. Pleas from Central and Eastern European leaders, missteps by Russian President Boris Yeltsin, and victory by the pro-expansion Republican Party in the 1994 U.S. congressional election all helped advocates of full-membership enlargement to win. The documents also reveal the surprising impact of Ukrainian politics on this debate and the complex roles played by both Strobe Talbott, a U.S. ambassador and later deputy secretary of state, and Andrei Kozyrev, the Russian foreign minister. Finally, the sources suggest ways in which the debate's outcome remains significant for transatlantic and U.S.-Russian relations today.