After the Soviet Union’s collapse in the early 1990s, the alliance took on a wider 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.
The alliance is also known by its French acronym, OTAN.
Which countries are members?
In addition to the United States and Canada, the countries that became part of NATO in 1949 were Belgium, Britain, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway and Portugal.
Since then, 19 more European states have joined: Albania, Bulgaria, Croatia, the Czech Republic, Estonia, Finland, Germany, Greece, Hungary, Latvia, Lithuania, Montenegro, North Macedonia, Poland, Turkey, Romania, Slovakia, Slovenia and Spain.
Finland’s membership adds one of Western Europe’s most potent militaries to the alliance. At the same time, NATO’s commitment to collective defense will now extend to a country that shares an 830-mile border with Russia.
Mr. Stoltenberg, who made the announcement about Sweden from Vilnius, said that President Recep Tayyip Erdogan had lifted his objections to Sweden’s entry and would take the country’s bid to his Parliament for ratification as soon as possible.