- How many divisions? -

NATO's military forces come from its member countries, put under a unified Allied command each time it launches a mission.

US forces are by far the biggest contingent. In Europe, there are some 79,000 US servicemen and women deployed, more than 48,000 of them in Germany, according to the Pentagon.

The only military equipment NATO itself owns is a fleet of sensor-loaded AWACS planes and drones.

With Sweden joining, the alliance will be able to count on 50,000 more troops -- around half of them reservists -- to swell its potential ranks to more than three million, according to several specialised institutes.

- Brussels HQ -

NATO was created from a Washington Treaty signed on April 4, 1949. Its mission at the time was to counter the threat posed by the Soviet Union and to affirm the principle of mutual defence of its members.

In a Madrid summit following Russia's all-out invasion on Ukraine in 2022, the alliance adopted a new strategic concept designating Russia as "the gravest threat to Euro-Atlantic security".

Its 12 founding nations (Belgium, Britain, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal and the United States) have been joined by others over succeeding years, up to Sweden's adhesion.

NATO's first headquarters was in London, then it was moved to Paris before being transferred to Brussels in 1966 where it remains today. Its SHAPE military command is in Mons, also in Belgium.

The head of the alliance since 2014 is Jens Stoltenberg, a 64-year-old former Norwegian prime minister. His term was extended in 2022 and runs to the end of October this year.

NATO's main decision-making body is the North Atlantic Council.

- Article 5 -

The core of NATO is Article 5 of its founding treaty which stipulates that an attack on one member is considered an attack on all.

It has been invoked just once to date: in 2001, in a show of allies' support for the United States the day after it was hit by the September 11 attacks.

NATO has carried out several missions over its history, including one in Afghanistan following the September 11, 2001 attacks. That came to an end with a hasty pull-out in September 2021 of the 10,000 personnel deployed at the time.

NATO is present in Iraq and notably in Kosovo, where it has 4,500 troops in its KFOR stabilisation force sent from 27 countries and currently under Turkish command.