(CNN) NATO has formally invited Montenegro to join the alliance, a move that's spurred threats from Russian officials at loggerheads with NATO over everything from Ukraine to Syria to Turkey's downing of a Russian warplane.

While it comes at a time of heightened tensions between NATO and Russia, it didn't happen out of the blue; it's the result of a process that began nine years ago.

"Today, we proudly receive a #NATO membership invitation," said Montenegro's President Milo Djukanovic, according to his government's Twitter feed . "This is a historic day for #Montenegro. The most important (since) the 2006 (independence) referendum."

#Montenegro thanks @NATO allies for the invitation to join the Alliance. pic.twitter.com/h0mPirbKej

As expected, this celebratory sentiment wasn't echoed in Russia.

"Moscow has always noted at various levels that the continuing expansion of NATO and NATO's military infrastructure to the East, of course, cannot but lead to response actions from the East, namely, the Russian side in ensuring security interests and supporting the parity of interests," Kremlin spokesman Dmitry Peskov said, according to the state-run Tass news agency

Peskov declined to say what Russian President Vladimir Putin will do about the Montenegro invitation, saying, "Now, there are other priorities."

But Viktor Ozerov, the chair of the Federation Council's Committee on Defense and Security, told another state news agency that Russia will end military and technical cooperation with Montenegro if its membership becomes official.

"Montenegro should recognize that a lot of programs that have been previously realized by it with Russia ... will be impossible in the context of its NATO accession," Ozerov told RIA Novosti.

Kerry: 'NATO is not a threat to anybody'

Twenty-eight countries , from the United States and Canada on one side of the Atlantic to a host of nations on the other, currently make up NATO. Twelve have been part of the bloc since its inception in 1949, in the thick of the Cold War, as a bulwark against the then-Soviet Union, and its membership has expanded periodically since

Its biggest single push came in 2004 with the addition of Bulgaria, Estonia, Latvia, Lithuania, Romania, Slovakia and Slovenia. Albania and Croatia joined NATO five years later. Like Montenegro, all those countries are in Eastern Europe and have historic ties to Moscow.

U.S. Secretary of State John Kerry took pains Wednesday to point out that Montenegro's possible accession to NATO "is not new," having been in the works for years.

Still, Wednesday's announcement comes at a particularly strained time in NATO-Russia relations.

The two sides have been at odds over Ukraine , where protesters pushed out the president about two years ago because of his closeness to Russia. The country has been boiling ever since, with Kiev and its Western allies -- including NATO , even though Ukraine isn't a member -- blasting Moscow for annexing Crimea and fomenting unrest elsewhere, while Russia says it favors popular determination for Ukraine and downplays its military's role in armed conflict.

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Another major area of contention is Syria. Putin has been one of Syrian President Bashar al-Assad's strongest (and few) allies in that country's bloody, years-long civil war, at a time when many, including U.S. President Barack Obama, have called for Assad's ouster and supported moderate rebels fighting him.

There have been many spillover effects from this conflict, the latest -- and most perilous to Russia-NATO relations -- coming when Turkey, a NATO member, shot down a Russian warplane . (Exactly where is a point of contention, with Moscow claiming the aircraft was struck in Syrian airspace and Ankara saying it was over Turkey. The nations also give conflicting reports on whether any warnings went out beforehand.)

That's why Montenegro joining NATO matters beyond that Balkan state: because it threatens to drive another wedge between Russia and the alliance. Kerry, though, said Wednesday that Moscow shouldn't worry.

"NATO is not a threat to anybody," he said after meetings at NATO's headquarters in Brussels, Belgium. "It's not an offensive organization. It is a defensive alliance. It is meant to simply provide security.

"And I don't think it's focused on Russia, per se, or anybody else."

Montenegro leader says move 'big' for Balkans region

Becoming an official member of NATO would be significant for Montenegro because, under the alliance's charter, any attack on it would be considered an attack on all NATO members. So if something were to play out the way it did in Ukraine -- where Russian troops are accused of taking part in armed conflict against government forces -- then NATO might be obliged to intervene on Montenegro's behalf.

No one is saying that will happen anytime soon. Still, the way Russian state media portrayed Wednesday's announcement speaks to Moscow's firm opposition.

The Tass report described what it called "a major propaganda campaign advertising NATO" by powers-that-be in Montenegro. Another story, from Sputnik news , referred to "ongoing anti-government protests with thousands of citizens gathering in ... the country's capital, Podgorica, to demand that Montenegro stay out of the U.S.-led NATO military bloc."

"Obviously, it is a political project not in the real interests of the citizens of Montenegro," Sputnik quoted a representative of the Socialist People's Party of Montenegro.

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U.S. Gen. Philip Breedlove, NATO's supreme allied commander for Europe, stressed Wednesday to CNN's Christiane Amanpour that "nations come to us and ask to be a part of NATO and we absolutely support every nation's individual right to seek its own individual destiny."

"The timing is right," Breedlove, the head of U.S. European Command, said of Montenegro's NATO invitation. "I don't think that we should allow Russia a veto over who wants to become a part of our alliance."

Montenegro is already involved in NATO's efforts in Afghanistan and has actively cooperated with the alliance in other ways.

The country has also enacted several reforms to meet the alliance's requirements, though Stoltenberg said NATO still expects more progress "on defense adaptation, on domestic reform, especially rule of law, and to continue to make progress in demonstrating public support for Montenegro's NATO membership."

Djukanovic, the Montenegrin Prime Minister, signaled Wednesday that his government is even more intent now on pursuing such reforms.

He also expressed hope the announcement would lead to broader changes elsewhere in his region.

"Inviting #Montenegro (is a) big day for W. Balkans," the Montenegrin government tweeted, citing Djukanovic. "I believe this is wind in (the) sails for reform and integration processes in region."