In Pics: Timeline of NATO expansion since 1949

1949: The North Atlantic Treaty Organization is founded to deter Soviet expansion and a revival of European militarism. The 12 original members are the United States, Canada, Britain, Belgium, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway and Portugal.

1952: Greece and Turkey join in the alliance's first expansion.

1955: West Germany joins NATO. In response, the Soviet Union and seven countries in Eastern Europe form the eight-nation Warsaw Pact.

1982: Spain becomes the 16th member of NATO.

1991: The Soviet Union collapses and the Warsaw Pact is dissolved.

1994: Finland and Sweden join NATO's Partnership for Peace program. The following year they join the European Union, effectively ceasing to be neutral, but remaining military nonaligned.

1999: Three former Warsaw Pact nations — the Czech Republic, Hungary and Poland — join NATO.

2001: Article 5 in the NATO treaty, which stipulates that an attack on any NATO member is an attack on all, is triggered for the first time after the 9/11 attacks on the United States.

2002: The NATO-Russia Council is formed to help NATO members and Russia to work together on security issues.

2003: NATO takes command of the International Security Assistance Force in Afghanistan (ISAF).

2004: The biggest NATO expansion to date as seven countries become members: Bulgaria, Romania, Slovakia, Slovenia, Estonia, Latvia and Lithuania. The latter three are the only former Soviet republics to have joined the alliance.

2009: Croatia and Albania become NATO members.

2011: NATO enforces a no-fly zone over Libya. Sweden takes part with fighter jets on reconnaissance missions.

2014: NATO suspends most cooperation with Russia after its annexation of Crimea.

2015: NATO ends the ISAF mission in Afghanistan. The alliance remains in Afghanistan to train local security forces until the Taliban takeover in 2021.

2017: Montenegro joins NATO.

2020: North Macedonia becomes NATO's 30th member.

2022: Sweden and Finland explore the possibility of NATO membership after Russia's invasion of Ukraine.