NATO declared Wednesday evening that if the terrorist attacks on New York and Washington turn out to be of foreign origin, they will be considered an attack on the entire alliance and the United States will be able to demand military assistance.
"The parties will take such action as it deems necessary, including armed force," NATO Secretary-General Lord Robertson told a news conference in Brussels.
The declaration would allow the United States to invoke Article V of the NATO charter, which declares an "armed attack" on any member to be an attack on all 18 members of the alliance. Originally intended to deter a Soviet invasion, it has never been invoked.
Although the solidarity principle was a founding tenet of the alliance in 1949, it "is no less valid" today, said Robertson.
"In the event of attacks ... each ally will assist [the United States] by taking such action as it deems necessary," a prepared statement read. "Accordingly, the United States' NATO allies stand ready to provide the assistance that may be required as a consequence of these acts of barbarism."
A separate meeting of European Union foreign ministers in Brussels ended with a similarly strong show of support for the United States.
The ministers said they would "spare no efforts to help identify, bring to justice and punish those responsible."
They also declared a day of mourning in all 15 EU nations for Friday and asked "all Europeans to observe three minutes of silence" on Friday at 6 a.m. EDT.
"We were all victims of this attack," Belgian Foreign Minister Louis Michel, who chaired the EU meeting, told reporters.
Belgium currently holds the six-month rotating presidency of the European Union, which is a chiefly economic and monetary association separate from, but largely overlapping in membership with, NATO. The United States is not a member.
In an exceptional move, NATO secretary-general Robertson attended the EU meeting.
"We have to make clear to the world that [the EU and NATO] stand together," he said. "We are two organizations that speak with one voice, one strong voice."
Fox News' Paul Wagenseil and the Associated Press contributed to this report