NATO, or the North Atlantic Treaty Organization, formed in 1949 while much of Europe was still struggling to rise above the devastation of World War II.
When the Soviet Union blocked Allied access to parts of Berlin in 1948, and Germany was split into two separate nations, the United States and several European nations responded by signing the North Atlantic Treaty.
The treaty stipulates that NATO member nations will respond collectively to an attack against any other member. Participating countries are not obligated to respond with military force, but can choose to support other members with political, economic or other forms of aid.
"NATO's essential purpose is to safeguard the freedom and security of its members through political and military means," the Brussels-based group's website states.
NATO has also expressed a commitment to "democratic values" and the sharing of resources that will enhance the defense and security of member nations by preventing conflict.
But the group's embrace of military action is unambiguous: "If diplomatic efforts fail, it has the military capacity needed to undertake crisis-management operations," the website states.
Currently, NATO membership stands at 28 nations. The 12 original members from 1949 are Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States.
In the years since those founding members signed, seven additional nations have joined NATO: Greece, Turkey, Germany, Spain, the Czech Republic, Hungary and Poland.
In 2004, one year after NATO took command of the coalition of nations fighting in Afghanistan, another set of countries joined: Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia. In 2009, Albania and Croatia gained membership.
Besides its 28 member nations, NATO has strategic partnerships with other countries through its "Membership Action Plan," which offers assistance and advice to nations seeking membership in NATO. Four countries are currently trying to join NATO through those plans: Georgia, Bosnia/Herzegovina, Montenegro and the Macedonian Republic.
The member nations of NATO are represented by a delegation, each with one presiding "permanent representative" who is generally a seasoned diplomat or military officer. These representatives form the North Atlantic Council, the primary administrative body within NATO.
The chair of the North Atlantic Council and the overall director of NATO is the Secretary General, who is appointed by consensus of NATO members. The Secretary General usually serves a term of four years, though that term may be extended.
The leader of NATO's military operations is the Supreme Allied Commander Europe, who has typically been an American military leader (the Secretary General has typically been a European).
The current NATO Secretary General is Anders Fogh Rasmussen, the former prime minister of Denmark. Jens Stoltenberg, the former prime minister of Norway, has been chosen to replace Rasmussen when his term ends in October 2014.
In the years following the 1991 breakup of the Soviet Union, NATO's mission has shifted somewhat in response.
In the Balkan conflicts that occurred in the former Yugoslavia during the 1990s, NATO troops performed their first wartime actions, which included shooting down Bosnian aircraft, bombing campaigns and other airstrikes.
Following the September 11 terrorist attacks in the United States, NATO troops were called into action to fulfill the group's charter, which states that an attack on one member is an attack on all. In 2003, NATO assumed control of military operations in the anti-terrorist occupation of Afghanistan.
NATO forces have also been active in anti-piracy efforts off the east coast of Africa, and in the 2011 civil war in Libya.
In the weeks following the controversial Russian annexation of the Ukrainian semi-autonomous region of Crimea, NATO has played a role in managing the diplomatic crisis, though neither Russia nor Ukraine are full-fledged members of NATO.