NATO foreign ministers have agreed to invite Montenegro to join the military alliance, swiftly prompting a threat from Russia of unspecified retaliatory measures.
"We congratulate Montenegro," NATO Secretary-General Jens Stoltenberg told foreign ministers at the alliance headquarters in Brussels on December 2.
Describing the decision as “historic,” he said “every nation has the right to decide its own path, its own security arrangements."
Stoltenberg added that the invitation "makes clear NATO keeps its door open" and bolsters "our vision of a Europe whole and at peace."
Montenegrin Prime Minister Milo Djukanovic said "this is a crown jewel of the long-standing national efforts and comprehensive reform processes launched in 2006," when the former Yugoslav republic voted in a referendum to end its union with Serbia.
Deputy Prime Minister Igor Luksic called the NATO invitation "a positive signal for all of the Western Balkans."
And Defense Minister Milica Pejanovic Djurisic described the move as “a confirmation of our great progress in the sectors of defense and other areas relevant to the alliance accession."
The tiny Adriatic state has a small military with about 2,000 active members.
In Moscow, President Vladimir Putin's spokesman said that Russia has repeatedly warned that "the continuing expansion of NATO and the military infrastructure of NATO to the east cannot fail to lead to actions in response from the east -- that is, from Russia."
The spokesman, Dmitry Peskov, declined to say what those actions might be, but said they would be aimed "to provide for [Russia's] security interests and support parity" between Moscow and the alliance.
U.S. Secretary of State John Kerry sought to dispel Moscow's concerns, saying that NATO is "not a threat to anyone," including Russia.
NATO "is a defensive alliance, it is simply meant to provide security," Kerry told a news conference in Brussels. "It is not focused on Russia or anyone else."
WATCH: Kerry Says NATO Expansion Not A Threat To Russia
Moscow has actively sought to discourage Montenegro from joining the Western military alliance, which took in Balkan nations Croatia and Albania in 2009 -- NATO's most recent expansion.
The Russian Foreign Ministry warned last month that Montenegro's entry into NATO would be "another blow to European security and to relations between Russia and NATO."
Moscow also has previously described NATO's extension into the Balkans as a "provocation" and warned that closer integration with Europe would not lead to prosperity for Montenegro.
State-run Russian news agency RIA cited a senior member of Russia's upper parliament house as saying on December 2 that Russia will end joint projects with Montenegro if it joins NATO.
Viktor Ozerov, head of the Federation Council's defense and safety committee, said the projects that could be axed included those in military areas.
NATO's invitation now to Montenegro to become its 29th member comes 16 years after the alliance bombed Montenegro during the Kosovo war, when it was still in a union with Serbia.
It also comes amid continuing high tensions over Ukraine and could be a signal to Russia that its actions there will not discourage NATO from further enlargement.
WATCH: Celebrations In Montenegro
Russia annexed Crimea from Ukraine after Kyiv sought closer ties with the West following the ouster of former President Viktor Yanukovych in February 2014 during the Euromaidan protests. Moscow is continuing to support separatists in eastern Ukraine whose war against Kyiv has killed more than 7,900 people.
Pro-Russian opposition groups in Montenegro had stoked unrest in recent months over the prospect the country could become a NATO member.
The right-wing New Serbian Democracy (NOVA) party took repeatedly to the streets of Podgorica to demand Djukanovic's resignation and either snap elections or an interim government.
In the most violent incident, some 5,000 demonstrators massed around the parliament hurled Molotov cocktails at police on October 24. Dozens of demonstrators and police were reported injured.
Djukanovic, who refused to step down, accused Russia and Serbia of instigating the turmoil.
Srdjan Milic, leader of the opposition Socialist People's Party, said NATO's invitation "represents an aggression [against] peace, stability, and security of citizens of our country."
In the run-up to NATO's invitation to Montenegro December 2, it had remained uncertain whether the alliance regarded the Balkan country as ready for membership.
The alliance had linked Montenegro's prospects to progress by the government on reforms to tackle corruption and improve the rule of law, as well as ensuring public support for joining the organization.
A public opinion poll conducted by the Podgorica-based Center for Democracy and Human Rights (CEDEM) on November 12 showed respondents evenly split, with 36.5 percent in favor of joining NATO and 36.2 against -- an insignificant difference given the poll's margin of error.
Although Montenegro has now been invited to join NATO, it can take up to 18 months for a country to formally accede. To do so requires approval by legislators in each of the 28 member nations. NATO diplomats say membership is likely to be cemented at an alliance summit in Warsaw in July 2016, Reuters reported.
Until NATO states ratify the move, Stoltenberg said Montenegro will be a nonvoting participant in meetings.
WATCH: Montenegro Invited To Join NATO
In a statement, the NATO foreign ministers encouraged Montenegro to intensify “efforts at political compromise and reform, in particular ensuring effective democratic dialogue, the rule of law, media freedom, and judicial independence.”
Luksic promised that Montenegro would continue carrying out reforms, including fighting corruption and improving the defense sector.
"We are fully aware that the invitation is not the end of the process, but the beginning of a new one," he said. "We're determined to constantly improve and work tirelessly, not to please others but to change our society for the better."
Serbia, which has close ties with Russia, is the only Balkan country not actively pursuing membership in the alliance.
Serbian Renewal Movement leader Vuk Draskovic, who wants Serbia to join NATO, welcomed the invitation for Montenegro.
"It is huge step for Montenegro today and a road map for Serbia for tomorrow," he told RFE/RL's Balkan Service in an interview in Belgrade.
Other potential candidates for NATO membership include the former Yugoslav republics of Bosnia-Herzegovina and Macedonia and the former Soviet republic of Georgia, whose NATO aspirations were one of the catalysts of a five-day war with Russia in 2008.
“We remain as committed as ever to the membership aspirations of these countries," Stoltenberg said on December 2. "We will do everything we can to assist them in achieving their goals, judging each aspirant country on its own merits.”
In their statement, the NATO foreign ministers called on Bosnia’s leadership to “continue demonstrating political will, particularly in pursuing reforms related to those aspirations.”
“Georgia’s relationship with the alliance contains all the practical tools to prepare for eventual membership,” the statement also said. It reiterated calls for the Russian military to withdraw from two breakaway Georgian regions, but it seemed unlikely to provide momentum for Georgia's membership talks.
“As we prepare for the Warsaw summit, we will explore new, practical ways to intensify efforts including through high-level political dialogue and increased cooperation, including in defense and strategic communications,” it said.