The North Atlantic Treaty Organization is a political and military alliance of 30 mostly European nations. The heart of the NATO alliance is Article 5, an agreement that an armed attack on one member will be viewed as an attack on all, and that they will defend one another.

NATO was founded in 1949 by 12 countries concerned that the Soviet Union would expand its political and economic system, called communism, beyond Eastern Europe. This was during a time called the Cold War, which ended with the collapse of the Soviet Union in 1991.

NATO now has 30 members. Although its membership is almost entirely European, Article 5 (the all-for-one defense pledge) has been invoked just once — after the Sept. 11, 2001, terrorist attacks on the United States. NATO also supported the United States in the war in Afghanistan.

NATO’s headquarters is in Belgium. Funding for its operations comes from members based on their national incomes. The United States, one of the world’s richest economies, pays more than other NATO countries.

NATO does not have its own armed forces. Instead it has a military command structure that works with the militaries of member countries in peacekeeping operations.

Countries wanting to join NATO must meet political, economic and military goals proving that they can contribute to NATO’s security as well as benefit from it. No country that has joined NATO has ever left it. Meanwhile, Ukraine’s bid for membership is not likely to advance while the country is at war, analysts say.

Some people confuse NATO with the United Nations. Both organizations focus on peacekeeping, but the United Nations, with 193 member countries, seeks cooperation in areas such as international law, human rights, the environment and social progress.