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The North Atlantic Treaty Organization, known as NATO, grew largely out of Cold War fears of Soviet aggression and expansionism, prompted by a Moscow-backed communist coup in Czechoslovakia, the Soviet blockade of Berlin that led to the Berlin Airlift, and other incidents. It was also meant to prevent the resurgence of nationalist militarism in Europe, and encourage political integration.

The United States, Canada, Belgium, Denmark, France, Iceland, Italy, Luxemburg, the Netherlands, Norway, Portugal, and the United Kingdom signed the initial treaty on April 4, 1949. Since then, the trans-Atlantic security arrangement has more than doubled in membership and changed its mandate significantly. Here are some highlights:

WHAT DOES NATO DO?

NATO was initially largely a political alliance but that changed quickly after the Soviets detonated an atomic bomb in 1949, and the Korean War in broke out in 1950. Those events prompted members to swiftly integrate forces and establish a centralized headquarters and commit to "safeguard the freedom, common heritage and civilization of their peoples, founded on the principles of democracy, individual liberty and the rule of law." A key provision of the treaty, the so-called Article 5, states that if one member of the alliance is attacked in Europe or North America, it is to be considered an attack on all of them. That effectively put Western Europe under the U.S. "nuclear umbrella." The head of the alliance is always a civilian secretary general, currently former Norwegian Prime Minister Jens Stoltenberg . NATO is led militarily by the Supreme Allied Commander Europe, who is also the commanding general of the Stuttgart-based U.S. European Command, currently Gen. Curtis Scaparrotti .

MUTUAL DEFENSE

The only time NATO's Article 5 mutual defense provision has been invoked was in support of the United States after the Sept. 11, 2001, attacks on New York and Washington. In response, the alliance activated AWACS reconnaissance flights over the U.S. for months, in operations that included 830 crew members from 13 NATO countries. It also launched maritime operations in the Mediterranean, and participated in U.S.-led efforts in Afghanistan, where it has led the mission since 2003.

WHO BELONGS TO NATO?

From the original 12 nations, NATO has grown today to an alliance of 29, and it is currently negotiating with several others. In its first expansion in 1952, NATO admitted Greece, Turkey and West Germany into the alliance. In response to NATO's growth and decision to include West Germany, the Soviet Union and its Eastern European client states formed the eight-nation Warsaw Pact, including East Germany, in 1955 with its own mutual defense agreement. The Warsaw Pact dissolved in 1991 following the fall of the Berlin Wall and the reunification of Germany, but NATO continued to expand. It now counts all seven non-Soviet former Warsaw Pact nations as part of its membership.

ALWAYS FRIENDS?

Though pledged to defend one another, NATO nations have not always seen eye-to-eye. Never was this more apparent than in 1966 when founding member France announced it was leaving NATO's integrated military command structure to pursue its own defense strategy, and asked the alliance to remove all Allied headquarters from its soil. That prompted the alliance to relocate from its initial base outside Paris to Belgium, where it remains today. Despite the move, France did not leave the alliance, emphasizing its commitment to its mutual defense pact while, at the same time, developing its own nuclear deterrent. Then French President Charles de Gaulle said the intention was to "modify the form of our Alliance, without altering its substance." France rejoined NATO's military structure in 2009.

2 PERCENT?

Much has been made of the alliance's goal that member nations spend at least 2 percent of gross domestic product on defense, due to U.S. President Donald Trump's regular contention that members are shirking their commitments. Reality is that In 2014, NATO members agreed to stop cutting their military budgets and set a goal of moving "toward" spending 2 percent of their gross domestic products on defense by 2024. NATO defines defense expenditure as payments by a government to meet the needs of its own armed forces, those of its allies or the alliance — not a payment of funds to NATO itself. According to this month's figures from NATO, the U.S. commitment is the highest with 3.5 percent of GDP, followed by Greece with 2.27 percent, Estonia with 2.14 percent and the United Kingdom with 2.1 percent. Latvia also meets the 2 percent goal, and Poland, Lithuania and Romania are expected to by year's end. None of the alliance's other members meet the target.

NATO NOW

NATO and the Warsaw Pact did not clash head-to-head during the Cold War, but the alliance has been busy in military engagements since the collapse of the Soviet Union. It says some 20,000 military personnel are currently engaged in NATO missions around the world, operating in Afghanistan, Kosovo and the Mediterranean. It conducted its first military intervention in Bosnia, implementing aspects of the Dayton Peace Agreement that marked the end of the 1992-1995 war. Its Implementation Force, or IFOR, was deployed in Bosnia in December 1995, followed by a Stabilization Force, or SFOR, which ended in December 2004. It took over leadership of the ISAF mission in Afghanistan since 2003 and currently leads the follow-on Resolute Support mission. It also supports the African Union in peacekeeping missions, has sent trainers to Iraq, assisted in counter-piracy missions in the Gulf of Aden and off the Horn of Africa, and enforced a no-fly zone over Libya, where it now has sole command and control of all military operations. It has also been involved in protecting public events like the Olympic Games in Greece in 2004, providing assistance in the aftermath of Hurricane Katrina in the U.S. in 2005, and helping Pakistan after a devastating earthquake in 2005.