The alliance is perhaps best defined by Article 5 of its founding document: "An armed attack against one [NATO member] ... shall be considered an attack against them all."
During the Cold War, this pledge of collective defense was aimed at "countering the threat posed at the time by the Soviet Union," NATO says. It was a warning that an attack on an ally in Europe would result in retaliation from the nuclear-armed U.S.
NATO's first secretary-general, Lord Hastings Lionel Ismay, said NATO existed to "keep the Soviet Union out, the Americans in, and the Germans down."
"NATO was a pillar of security during the Cold War, serving as a European counterbalance to the Soviet Union," said Rebecca Lissner, a research fellow at the University of Pennsylvania's Perry World House, a global policy research center.
The only time NATO triggered Article 5 was after the terror attacks on 9/11. In response, NATO aircraft patrolled U.S. skies and its ships were dispatched to the Mediterranean Sea to counter terrorism and trafficking.
The 12 original signatories of the North Atlantic Treaty were the U.S., Canada, the United Kingdom, France, Italy, Portugal, Norway, Denmark, Iceland, Belgium, the Netherlands and Luxembourg.
Greece and Turkey joined in 1952, followed by West Germany in 1955 and Spain in 1982.
After the fall of the Soviet Union, the Czech Republic, Hungary and Poland became members in 1999, then Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia in 2004, and Albania and Croatia in 2009.
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
One allegation is that NATO is guilty of "antagonizing Russia" and "extending security guarantees to states that would be hard to defend ... against a Russian attack," according to Lissner at Perry World House.
While these critiques have "some legitimacy," Lissner believes the answer is not dissolution but modernization.
"The challenge," she says, is for NATO to balance "traditional security threats," such as Russia and nuclear-based concerns, with nontraditional dangers such as terrorism and cyberwarfare.