(CNN)Here's a look at the North Atlantic Treaty Organization (NATO), headquartered in Brussels, Belgium.
The organization's charter states that the signing parties will "seek to promote stability and well-being in the North Atlantic area," and will "unite their efforts for collective defense and for the preservation of peace and security."
April 4, 1949 - Established when 12 nations sign the North Atlantic Treaty in a ceremony in Washington, D.C.
2009-present - The current secretary general is Anders Fogh Rasmussen of Denmark.
28 Member Countries:
Czech Republic (1999)
Germany (1955, as West Germany)
United Kingdom (1949)
United States (1949)
April 4, 1949 - The 12 nations of Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States sign the North Atlantic Treaty in Washington, D.C.
July 25, 1950 - First meeting of NATO Council Deputies in London. U.S. Ambassador Charles M. Spofford is elected permanent chairman.
December 19, 1950 - General Dwight Eisenhower is appointed the first supreme allied commander.
April 2, 1951 - Allied Command in Europe becomes operational with Supreme Headquarters Allied Powers Europe (SHAPE) in Roquencourt, near Paris.
March 12, 1952 - Lord Ismay is named the first secretary general of NATO and appointed vice chairman of the North Atlantic Council.
April 10, 1952 - Allied Command Atlantic (ACLANT) becomes operational, headquartered in Norfolk, Virginia.
April 16, 1952 - NATO establishes its provisional headquarters in Paris at the Palais de Chaillot.
April 28, 1952 - First meeting of the North Atlantic Council in permanent session in Paris.
May 6, 1952 - West Germany joins NATO.
May 14, 1955 - The Soviet Union and Eastern Bloc countries form the Warsaw Pact in response to West Germany joining NATO.
August 13, 1961 - The Berlin Wall is erected.
October 22 -23, 1963 - NATO and the United States demonstrate the size and speed of emergency forces with 14,500 U.S. troops flown into West Germany for maneuvers.
March 10, 1966 - France formally announces intentions to withdraw from the military structure of NATO, accusing the United States of having too much influence in the organization.
March 31, 1967 - Opening ceremony of new NATO headquarters in Casteau, near Mons, Belgium in Brussels.
August 14, 1974 - Greece, angered at NATO's response to the Turkish invasion of Cyprus, withdraws from the military arm of NATO.
October 20, 1980 - Greece rejoins the NATO military structure.
May 30, 1982 - Spain joins NATO.
November 9, 1989 - The Berlin Wall comes down.
October 3, 1990 - Germany is reunified after 45 years. With German reunification, East Germany leaves the Warsaw Pact and is incorporated into NATO.
February 11, 1991 - The Warsaw Pact is dissolved.
December 13, 1991 - For the first time, the Soviet Union takes part in meetings at NATO as part of the North Atlantic Cooperation Council (NACC).
December 21, 1991 - Eleven of the republics of the former Soviet Union create a new Commonwealth of Independent States.
December 25, 1991 - The Soviet Union is officially disbanded with the resignation of Mikhail Gorbachev as president and supreme commander-in-chief of Soviet Forces.
February 28, 1994 - NATO forces shoot down four Bosnian Serb planes violating the U.N.-imposed no-fly zone. It is NATO's first military engagement in almost 50 years of history.
November 21, 1995 - After the Dayton Peace Accords, the war in Bosnia Herzegovina ends.
December 20, 1995 - The United Nations turns over military operations command to NATO's Implementation Force (IFOR).
January 13, 1996 - Russian troops deployed to support IFOR in Bosnia.
May 22, 1997 - NATO and the Russian Federation sign a security and cooperation pact, the "Founding Act" which establishes a NATO-Russia Permanent Joint Council (PJC).
March 24, 1999 - NATO launches air strikes against Yugoslavia to end Serbian aggression in the Kosovo region.
April 4, 1999 - 50th anniversary of founding of NATO, celebrated in various countries throughout the year.
August 22, 2001 - Operation Essential Harvest, the disarming of ethnic Albanians in Macedonia, begins.
May 14, 2002 - The last meeting of the NATO-Russia Permanent Joint Council
May 28, 2002 - NATO and Russia form the NATO-Russia Council (NRC), which makes Russia an associate member of the organization. The NRC replaces the Permanent Joint Council (PJC) that was established in 1997 by the NATO-Russia Founding Act.
November 21-22, 2002 - During the Prague Summit, NATO invites seven former Eastern Bloc countries, Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia, to discuss entry into the organization.
December 4, 2002 - Deputy Defense Secretary Paul Wolfowitz speaks before NATO in Brussels and requests that member nations contribute forces to a potential campaign in Iraq.
January 22, 2003 - France and Germany block discussion on war preparations submitted by the United States. The U.S. proposal included provisions for Turkey's defense, the use of NATO equipment, and NATO's postwar role in Iraq.
January 23, 2003 - Secretary General Lord Robertson announces his intention to step down in December.
February 10, 2003 - France, Germany and Belgium block a U.S. request that NATO provide Patriot missiles, Airborne Warning and Control System (AWACS) aircraft, and other equipment to Turkey. The United States had made the request anticipating that Iraq will retaliate against Turkey in the event of war. Turkey invokes article IV of the NATO charter, which requires the organization as a whole to discuss security threats to any member nation.
February 11, 2003 - A meeting to discuss the standoff over Turkey's defense preparations ends after 20 minutes with no resolution.
February 16, 2003 - NATO comes up with three defensive plans for Turkey, in the event of a U.S. war with Iraq:
- deployment of NATO Airborne Warning and Control System (AWACS) aircraft;
- NATO support for the deployment of theatre missile defences for Turkey;
- NATO support for possible deployment of Allied chemical and biological defences. (from NATO website)
February 19, 2003 - NATO deploys defensive assistance to Turkey in the form of missiles, chemical and biological defense mechanisms, and aircraft, in the event war with Iraq presses forward.
January 5, 2004 - Jaap de Hoop Scheffer of the Netherlands becomes the new secretary general.
March 29, 2004 - NATO is expanded from 19 to 26 members when the seven nations Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia, and Slovenia, join in an accession ceremony in Washington, DC. All are former communist states in Eastern Europe.
April 2, 2004 - First meeting of the NATO-Russian Council with 27 members.
August 10, 2004 - NATO Airborne Warning and Control Systems (AWACS) begin patrolling Greek airspace prior to the Olympic and Paralympic games. NATO's presence at the Olympics is nicknamed Distinguished Games and includes AWACS and the Multinational Chemical Biological Radiological Nuclear Task Force (MN CBRN TF).
April 2-4, 2008 - NATO leaders hold a summit in Bucharest, Romania. Croatia and Albania are invited to join the alliance.
June 17, 2008 - French President Nicolas Sarkozy announces France will soon rejoin NATO's military command, 40 years after it left.
April 3-4, 2009 - The 23rd NATO summit also marks NATO's 60th anniversary. Events are held in Strasbourg, France and Kehl, Germany. During the summit, France rejoins NATO's military command.
August 1, 2009 - Former Danish Prime Minister Anders Fogh Rasmussen takes office as the 12th secretary general of NATO.
November 19, 2010 - NATO adopts the Strategic Concept "Active Engagement, Modern Defence" for the next 10 years.
March 24, 2011 - NATO takes command of enforcing a no-fly zone imposed on Libya by the United Nations.
March 29, 2011 - Council of Europe rules NATO, among others, responsible for the 63 deaths from among 72 African immigrants left adrift for two weeks while attempting to reach European shores from Libya in March 2011.
May 19, 2012 - Demonstrators take to the streets of Chicago prior to the start of the NATO summit. Anti-NATO protests near Mayor Emanuel's home focus on the cost of the summit to the city and city budget cuts to mental healthcare.
May 20-21, 2012 - The 25th Summit is held in Chicago. During the summit, NATO accepts President Barack Obama's timetable to end the war in Afghanistan by 2014.
March 5, 2014 - In regards to the crisis in Ukraine, Secretary General Anders Fogh Rasmussen announces that NATO has decided to "put the entire range of NATO-Russia cooperation under review" to send "a clear message Russia's actions have consequences."
December 2, 2015 - NATO extends an official invitation to Montenegro to join the alliance.