From the perspective of the cold war, it seemed incongruous that the North Atlantic Treaty Alliance (NATO) would invite Poland, Hungary, and the Czech Republic to join. NATO, after all, had been formed as a bulwark against those countries and others, most notably the Soviet Union, in the Warsaw Pact. But the cold war had ended with the fall of eastern European communism in the early 1990s, and the new Europe that emerged was one in which all countries could join forces to protect one another and preserve the peace. Accordingly, NATO extended the invitation to its former enemies on July 8, 1997, with full membership to begin in time for the organization's 50th anniversary in 1999.
The Soviet Union, which began World War II on the same side as its fellow totalitarian state, Nazi Germany, might well have remained a Nazi ally if the Germans had not invaded it in 1941. The entry of the United States to the war later that year ultimately turned the tide of the European conflict, as America and Great Britain joined forces with the Soviets to defeat the Nazis. Though there was talk of common aims during the war, the alliance of liberal democracies and Josef Stalin's brutal regime was in actuality a marriage of convenience. This became apparent after the war's end, when Stalin used his troops, already present in eastern Europe, to assist in the establishment of communist regimes in six nations: East Germany, Poland, Hungary, Czechoslovakia, Romania, and Bulgaria. (Albania and Yugoslavia also adopted communist governments, in part through the help of Stalin, but they would not remain aligned with Moscow.)
Fearing the threat of Soviet expansion westward, particularly in Germany, the liberal democracies of western Europe and North America signed the North Atlantic Treaty, also known as the Washington Treaty, on April 4, 1949. The treaty provided for mutual defense in the event of aggression against any member state, and established NATO to coordinate the actions of members. The 12 original signatories were Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, The Netherlands, Norway, Portugal, the United Kingdom, and the United States. In February 1952, NATO admitted Greece and Turkey, and in May 1955, West Germany.
The Soviet bloc countered this last move by forming the Warsaw Pact on May 14, 1955. The Warsaw Pact joined the Soviet Union with its six European satellites (as well as Albania, which withdrew in 1968) in a mutual-defense alliance intended to mirror that of NATO. Beginning in 1989, however, popular uprisings eventually deposed communist governments in eastern Europe. East Germany withdrew from the Warsaw Pact in 1990, after it reunited with West Germany, and on July 1, 1991, the remaining Warsaw Pact signatories formally declared their alliance "nonexistent".
Recognizing that the security needs of Europe had changed dramatically in recent years, NATO members voted at a summit in Madrid (Spain joined NATO in 1982) to extend membership to three former Warsaw Pact countries: the Czech Republic (formed in 1993, when the 75-year union with Slovakia was dissolved), Poland, and Hungary. The three would become full members in March 1999, just in time for the organization's 50th anniversary.
With eastern Germany, this effectively extended NATO membership to four of the six former Soviet satellites. NATO members expressed a willingness to consider applications for membership by other eastern European nations, but this move was not without controversy. A number of American leaders--President Bill Clinton among them--viewed with concern the possibility that too large a membership could make for a cumbersome organization. Though Clinton welcomed the admission of the three countries invited in 1997, noting that NATO had "erased an artificial line drawn across Europe by Stalin after the Second World War," he himself drew the line at these three new members, and counseled against admitting new ones. Nonetheless, at its April 1999 summit in Washington, D.C., NATO established a "Membership Action Plan" to assist nations in preparing for membership.