The North Atlantic Treaty Organisation, known as Nato, grew largely out of Cold War fears of Soviet aggression and expansionism following a communist coup in Czechoslovakia, the Soviet blockade of Berlin and other incidents.

It also was meant to prevent the resurgence of nationalist militarism and to encourage political integration in Europe.

The United States, Canada, Belgium, Denmark, France, Iceland, Italy, Luxemburg, the Netherlands, Norway, Portugal, and Britain 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 initially was largely a political alliance. That changed quickly after the Soviets detonated an atomic bomb in 1949 and the Korean War broke out in 1950.

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The events prompted members to establish a centralised headquarters, to commit joint military resources and to commit to "safeguard the freedom, common heritage and civilisation 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.

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 US European Command, currently General Curtis Scaparrotti.

Mutual defence
The only time Nato's Article 5 mutual defence provision has been invoked was in support of the US after the September 11, 2001, attacks on New York and Washington.

Who belongs to Nato?
Nato has grown from the original 12 nations to an alliance of 29. Several other nations are in membership negotiations.

The first expansion, in 1952, admitted Greece, Turkey and West Germany into the alliance. In response to Nato's growth and decision on West Germany, the Soviet Union and its Eastern European client states in 1955 formed the eight-nation Warsaw Pact. East Germany was included in the pact, which had a mutual defence agreement of its own.

The Warsaw Pact dissolved in 1991. Nato now counts all seven non-Soviet former Warsaw Pact nations as alliance members.

NATO Secretary-General Jens Stoltenberg speaks at a press conference during a summit of heads of state and government at NATO headquarters in Brussels, Belgium. Photo / AP
NATO Secretary-General Jens Stoltenberg speaks at a press conference during a summit of heads of state and government at NATO headquarters in Brussels, Belgium. Photo / AP

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 defence strategy and asked the alliance to remove all Allied headquarters from its soil.

The alliance relocated from its initial base outside Paris to Belgium, where it remains today. Despite the move, France did not leave Nato, emphasising its commitment to the mutual defence pact. But France continued developing its own nuclear deterrent. France rejoined Nato's military structure in 2009.

2 per cent?
Much has been made of the alliance's goal for member nations to spend at least 2 per cent of gross domestic product on defence. US President Donald Trump routinely contends that most members are shirking their commitments by failing to get there.

The reality is that in 2014, Nato members agreed to stop cutting their military budgets and set a goal of moving "toward" spending 2 per cent of their gross domestic products on defence by 2024.

Nato defines defence expenditure as payments by a government to meet the needs of its armed forces, those of its allies or the alliance — not a payment to Nato itself.

According to Nato figures, the US commitment is the highest with 3.5 per cent of GDP, followed by Greece with 2.27 per cent and Estonia with 2.14 per cent.

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.

Nato 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 Stabilisation Force, or Sfor, which ended in December 2004.

The alliance also took over leadership of the Isaf mission in Afghanistan in 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.

It has been involved in protecting public events and assisted in the aftermath of natural disasters.