Over Russia's angry objections, NATO agreed Thursday to expand for only the seventh time in its history, inviting the Balkan nation of Montenegro to become its 29th member.
The decision is still subject to formal approval by the U.S. Senate and the alliance's other national parliaments.
NATO Secretary-General Jens Stoltenberg said it was the "beginning of a new secure chapter" in the former Yugoslav republic's history.
Montenegro's prime minister, Milo Dukanovic, who attended the signing of an accession protocol at NATO headquarters in Brussels, said his country, bombed by NATO warplanes 16 years ago, would stand "shoulder to shoulder" with the other members of the U.S-led alliance.
"You can count on us at any time," said Dukanovic.
Russia has accused NATO of trying to encircle it and friendly nations like Serbia, and vowed to do what's necessary to defend its national security and interests.
In April, the NATO-Russia Council held its first meeting in almost two years, "a session some hoped would signal a thaw in relations," as The Christian Science Monitor reported.
"The two entities have had an increasingly strained and complicated relationship, as a series of events has put them at loggerheads," the Monitor's Jason Thomson wrote at the time, listing "the war in Syria, Russia's alleged incursion into Ukraine and its annexation of Crimea, and, most recently, the harassment of US military assets by Russian jets in the Baltic Sea."
On Monday, Sergei Zheleznyak, a prominent member of the Russian parliament, said his country would have to alter its relations with Montenegro, historically close to Russia, if it joined NATO without holding a national referendum.
"We would have to change our policy in regard to this friendly country," Zheleznyak said. "If NATO military infrastructure were placed there, we would have to respond by limiting our contacts in economic and other spheres."
Other Russian officials have said their country could ban some imports from Montenegro and levy other trade sanctions.
The signing ceremony at NATO headquarters for Montenegro's membership invitation coincided with the start of a NATO foreign ministers' meeting, and Secretary of State John Kerry signed the document on behalf of the United States.