See map of which countries are NATO members — and learn how countries can join

President Biden is meeting with other world leaders at the NATO summit in Lithuania this week, where member nations are discussing Sweden's admission into their group. NATO recently grew to 31 nations, with the addition of Finland earlier this year, and could now grow to 32 with the inclusion of Sweden. Which countries are part of NATO, and how do they become members?

NATO, the North Atlantic Treaty Organization, formed in 1949 when the 12 founding nations signed the Washington Treaty. Central to the treaty is Article 5, a pact among member nations to provide mutual assistance against what was then the Soviet Union if it tried to extend its control over Europe.