NATO officially invited Montenegro to become its 29th member Thursday, angering Russia, which already is threatening sanctions against what it calls a "friendly country."
NATO foreign ministers signed the Accession Protocol in Brussels, as Montenegrin Prime Minister Milo Dukanovic looked on.
All 28 legislative bodies, including the U.S. Senate, must ratify the protocol before Montenegro can join.
But NATO Secretary General Jens Stoltenberg said he expects to soon see 29 flags flying outside the organization's Brussels headquarters.
"Montenegro has already been contributing to NATO, EU and UN operations, promoting regional cooperation in the Balkans, and implementing major reforms," Stoltenberg said.
"Membership will give Montenegro the ability to help shape NATO policy. It will bring more stability and security to the region ... and it will be a clear sign that NATO's door remains open to partners that share and promote our values."
Russia, which feels threatened by NATO expansion right up to its borders, is threatening to "change its policy ... to this friendly country" if Montenegro becomes a new member.
A foreign ministry spokeswoman said this could include limiting Russian economic and other contacts with Montenegro.
She called NATO expansion another "attempt to change the military political landscape in Europe, especially in the light of the alliance's course to restrain our country."
NATO has increased its military presence across Europe, including the heavily Russian populated Baltics, since Russia annexed Ukraine's Crimean Peninsula in 2014.
Montenegro was a republic in the now-dismantled Yugoslavia, a country that was inside the Soviet-dominated Eastern Bloc during the Cold War.
It was part of a federation with Serbia after Yugoslavia broke up. Montenegro eventually became an independent country in 2006 and has been relatively stable compared with the ethnic strife that occasionally rocks other former Yugoslav republics.
Montenegro has contributed to NATO missions in Afghanistan.
NATO last expanded in 2009, when Albania and Croatia became members.