The treaty that gave the alliance its name has 14 articles, which all NATO members must abide by. One of the most important articles of the treaty is article 5, which declares that an attack on one member state is an attack on all.

This article placed Western Europe under American protection in the face of a Soviet Union which cemented its domination over Central and Eastern Europe and seemed only to grow in power and ambition.

After the collapse of the Soviet Union in the early 1990s, the alliance took on a broader role. NATO forces – made up of troops volunteered by member states – operated as peacekeepers in Bosnia in the 1990s and bombed Serbia in 1999 to protect Kosovo, where the alliance still has troops.

Which countries are part of NATO?

In addition to the United States and Canada, 10 other countries became members of NATO in 1949, including: Belgium, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal and Great Britain.

Since then, 18 other European powers have joined: Albania, Bulgaria, Croatia, Czech Republic, Estonia, Germany, Greece, Hungary, Latvia, Lithuania, Montenegro, Macedonia North, Poland, Turkey, Romania, Slovakia, Slovenia and Spain.

Other European countries have over the years avoided joining NATO, including Sweden and Finland.