Nato has invited the Balkan nation of Montenegro to become its 29th member, agreeing to expand for only the seventh time in its history despite Russia's angry objections.
The decision is still subject to formal approval by the US Senate, the alliance's 27 other national parliaments and Montenegro's parliament.
Nato secretary-general Jens Stoltenberg said it was the "beginning of a new secure chapter" in the former Yugoslav republic's history.
He and US Secretary of State John Kerry hailed the move as proof that Nato is committed to its "open door policy" of expansion despite opposition from Russia or any other country.
"Montenegro's accession underscores once again our determination to be able to make membership decisions that are free from outside influences, and underscores our resolve to stand together against any kind of threat," Mr Kerry said.
Montenegrin prime minister Milo Djukanovic attended the signing of an accession protocol at Nato headquarters in Brussels on Thursday. He said his country, bombed by Nato war planes 16 years ago, would stand "shoulder to shoulder" with the other members of the alliance.
"You can count on us at any time," he said.
But Russia has accused Nato of trying to encircle it, and Russian allies like Serbia have vowed to do what is necessary to defend its national security and interests.
"Yet another Nato attempt to change the military political landscape in Europe, especially in light of the alliance's course to restrain our country, inevitably affects Russian interests and forces us to react accordingly," Russian foreign ministry spokeswoman Maria Zakharova said.
Sergei Zheleznyak, a prominent member of the Russian parliament, said his country will have to alter its relations with Montenegro, which is historically close to Russia, if it joined Nato without holding a national referendum.
Other Russian officials have said Moscow 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 designed to set the stage for the alliance's summit in Warsaw this July.
Nato now faces simultaneous security challenges from Russia and from armed Islamic extremist movements in the Middle East and North Africa, and has been progressively devising a response.
Mr Stoltenberg said ministers agreed on Thursday to send an assessment team to Iraq to explore the possibility of Nato training Iraq's military in the country to help it better fight the Islamic State extremist group. Nato has already trained hundreds of Iraqi officers in Jordan.
Mr Stoltenberg said Nato is also considering aiding the US-led coalition combating IS by supplying command and control aircraft that could fly over Turkey or in international airspace and peer into Iraq and Syria.
And he said Libya's new UN-brokered government will send experts to Brussels to explore how Nato might help Libya rebuild its shattered defence and security apparatus to fight a local Islamic State affiliate.
"Nato is already doing a great deal with partners and for partners," Mr Stoltenberg said. "But we can and should do more. Because projecting stability in our neighbourhood is also about preserving security here at home."
Asked by reporters how long it will take for Montenegro to become a fully-fledged member, Mr Stoltenberg said he could not predict how fast legislators in Nato member nations will act, but that ratification of the accession protocols took about a year in the last expansion round.