Officially created by the Washington Treaty of April 4, 1949, the North Atlantic Treaty Organization sprang from European countries' desire for U.S. security guarantees against the westward expansion of communism and Soviet domination.
Amid the ruins of World War II, the Soviet Union was drawing its Iron Curtain across Central and Eastern Europe. To counter the Soviet threat, five European countries signed the Brussels Treaty in March 1948 -- creating the Western European Union.
Robert Walter, president of the European Security and Defense Assembly of the Western European Union, says that NATO could not have been created without the Brussels Treaty paving the way.
"Sixty-one years ago was the Treaty of Brussels, which basically brought together the Benelux countries, France, and the United Kingdom in an alliance that was a forerunner of NATO by just one year," Walter says. "We built on that with the Washington Treaty in 1949 -- which then encompassed the United States and Canada. And we've gone on from there as Europe has evolved in the post-World War II environment."
Indeed, one of the aims of the Brussels Treaty was to convince the United States to join with Europeans in a defensive alliance. With the signature of the Washington Treaty, the European alliance expanded to include Canada, Denmark, Iceland, Italy, Norway, Portugal -- and the United States.
One key section of the treaty fulfilled the European countries' hopes for a U.S. security guarantee against a Soviet invasion of Western Europe. It was Article 5: a promise that all NATO allies will come to the aid of any individual NATO country that is attacked.Reshaping The Alliance
After the Soviet-supported North Korean invasion of South Korea in June 1950, NATO leaders feared a Soviet attack on West Germany would be next. That is when they reshaped the alliance into a military organization with an integrated command structure.
In 1951, NATO established the Supreme Headquarters for Allied Powers in Europe (SHAPE). The allies chose U.S. Army General Dwight D. Eisenhower to be the first supreme allied commander in Europe. The next year, NATO bolstered its southeastern flank with its first enlargement -- Greece and Turkey.
In 1955, an agreement was reached in Paris on West Germany's membership. Walter describes it as a pivotal development that helped facilitate growing unity within Western Europe.
"The bringing of [West] Germany into the alliance was very, very significant indeed. And the creation, in parallel, of the economic developments of the European Community -- and then the European Union -- I think was also significant," Walter says.
To the chagrin of France, however, U.S. influence over NATO continued to grow during the 1950s.
Tensions between the Soviet Union and the alliance also rose as NATO shifted its reliance on conventional forces to a strategy of nuclear deterrence.
In the summer of 1961, mounting anxiety between East and West led to the creation of the Berlin Wall. At the same time, Washington tried to discourage France and West Germany from developing their own nuclear arsenals by proposing a European nuclear force.
But U.S. control over nuclear weapons in Europe was unacceptable to French President Charles de Gaulle. In 1966, de Gaulle withdrew France from NATO's military command structures -- forcing NATO to move its headquarters to Belgium.
De Gaulle's decision marked the start of a new phase in NATO history. A small group of NATO countries, led by Belgium, recommended a policy of detente alongside defense. Meanwhile, on the initiative of West German Chancellor Willy Brandt, East-West relations began to improve.
As the first part of the Strategic Arms Limitation (SALT) agreements were worked out in 1972, internal divisions surfaced again within NATO -- with some Europeans worried the United States and the Soviet Union would work out deals on their own.
Then, as Western Europe was lowering its defense expenditures, the Soviet Union began to deploy SS-20 nuclear missiles that were aimed at European cities.
NATO responded in 1979 by deciding to deploy ground-launched cruise and Pershing II missile systems in Europe. Those deployments began in 1983 and marked an intensification of the nuclear arms race.
Arms-control talks resumed two years later as Soviet leader Mikhail Gorbachev expressed interest in a new relationship with NATO, and as he launched his policies of glasnost and perestroika.Change In Direction
Neither Washington nor Europe anticipated the beginning of the collapse of the communist system that began in 1989 with the fall of the Berlin Wall and climaxed in 1991 with the collapse of the Soviet Union.
Former NATO spokesman Jamie Shea, who now directs policy planning in the private office of the NATO secretary-general, says that the fall of the Berlin Wall was a revolutionary moment that "seemed to spell the end of NATO."
"Why would you need NATO if there was no longer an adversary?" Shea says. "But in reality, the end of the Cold War brought about a situation where NATO discovered a new role for itself in terms of defense reform and security sector reform, helping these former communist countries to get back on their feet and expand the zone of peace and stability in Europe."
At the same time, the end of the Cold War also meant new threats to European stability -- regional and ethnic conflicts in the former Soviet Union and in the Balkans. It was then that NATO evolved from a purely defensive alliance into one that carried out offensive operations beyond its own borders.
NATO's first shots in a war were fired in 1995, when three weeks of air strikes were launched against Bosnian Serb forces that had attacked UN-designated "safe areas" in Bosnia.
Michael Zantovsky, the Czech ambassador to the United States at the time, notes that those attacks came only after European diplomats had failed for years to broker an end to the fighting in Bosnia and Croatia.
"All of a sudden, NATO became the only game in town that could effectively stem the tide of nationalism which threatened to engulf the countries emerging from decades of totalitarian rule -- in particular, in the Balkans," Zantovsky says.
"I think that we should never forget that it was NATO that stopped the fighting in the Balkans," he continues. "We can argue about the way it was done. We can argue about the force that was used. But it is not arguable that it was NATO who stopped it."
Former U.S. Secretary of State Madeleine Albright says that NATO's decision to launch air strikes on Serbia and Montenegro in 1999 had been heavily influenced by the earlier Balkan wars.
Albright was in office during Serbian forces' crackdown on ethnic Albanians in Kosovo. "I had watched that it had taken us too long to get into Bosnia. And so we took NATO to war in a much longer 78-day war," Albright says. "It was very significant because it was on behalf of freedom for people that were being pushed from their houses or killed."
Albright adds: "It showed the validity of the new mission for NATO. I hope that there were many lessons that came out of the Kosovo campaign that are useful in a different region now -- in Afghanistan. But it is part of the same post-Cold War story -- when an alliance that was set up to be anti-Soviet was, in fact, relevant in the end of the 20th century and beginning of the 21st."
Indeed, the terrorist attacks against the United States on September 11, 2001 -- which ultimately brought NATO forces into Afghanistan -- also were a historic milestone for the alliance. In response to those attacks, NATO invoked Article 5 of the Washington Treaty for the first and only time in its history.
"Article 5 was always based on the assumption that somebody would try to seize your territory and send a large conventional military force to cross your border," Jamie Shea says. "That's not the way it happened on September 11 when it was the United States -- the country which was providing the Article 5 guarantee to Europe -- which was itself attacked."
"It was Europe, paradoxically, which offered assistance to the United States," Shea continues. "It showed that NATO would have to be prepared to deal with a completely different set of security challenges in the 21st century. And it showed that although Article 5 is still sacrosanct, it might be applied in different ways -- for example, the ability to respond in places like Afghanistan. So it caused a process of rethinking in the alliance."
Michael Clarke, director of the London-based Royal United Services Institute for Defense and Security Studies, says there is another important lesson to be found in NATO's recent history. The alliance has undergone two successive expansions since 1999 -- adding Poland, Hungary, the Czech Republic, and the Baltic countries to its ranks -- and has just welcomed Croatia and Albania this year. Such growth, he says, is a reminder that successful alliances must have the capacity to adapt.
"After 1989 and 1991, I think the enlargement of NATO was a natural and correct reaction. If NATO had not enlarged, it would have been logical to have disbanded itself," Clarke says. "It had very little rationale, unless it enlarged when the Cold War was over, to represent those values and commitments which it had represented during the Cold War." RFE/RL's Bruce Jacobs contributed to this report