NATO's membership offer to Montenegro has set off a firestorm of protest from Moscow that might seem outsized considering the small scale of the Balkan country and its minimal military force.



Montenegro's army has about 2,000 soldiers, and its navy has just two active frigates.



But if the Kremlin's angry response seems out of proportion to what NATO stands to gain, it may make more sense given the unspoken message the alliance's invitation sends: that all nations should be free to choose their alliances, without Moscow's interference.

"One of the key arguments that has persuaded NATO member states to issue the invitation is precisely the message to Moscow that Russia cannot divide Europe into spheres of influence," says Jonathan Eyal, international director at the Royal United Services Institute (RUSI) in London.



He notes that the membership offer to Montenegro comes a year after Russia intervened in Ukraine in response to Kyiv seeking closer ties with the West. Moscow annexed Crimea after the ouster of President Viktor Yanukovych in February 2014, and it continues to support pro-Russian separatists who have seized parts of eastern Ukraine in a conflict that has killed more than 7,900 people.



Russia's ongoing interference in Ukraine has caused a wave of uneasiness among NATO states in Russia's immediate neighborhood and NATO has responded by increasing its presence in many of them.

Now, the NATO invitation for Montenegro, which Moscow had actively sought to dissuade from joining the alliance, appears intended to reassure both NATO members themselves and would-be members in Eastern Europe and the Balkans that the alliance is not intimidated by Moscow.



"For NATO, it gives a stronger sense of having a security reach in this part of Europe and sends a signal to other countries in the Western Balkans that NATO membership is a possibility for them in the future," says Judy Dempsey, a senior associate at Carnegie Europe and editor in chief of the Strategic Europe blog.



In making the invitation on December 2, NATO Secretary-General Jens Stoltenberg said that Bosnia, Macedonia, and Georgia were also making progress toward NATO membership. After NATO invited Georgia in 2008 to step up cooperation toward eventual membership, Russian forces drove deep into the ex-Soviet republic in a five-day war and Moscow recognized two breakaway Georgian regions as independent states.



The question now is how Russia will answer NATO's message.



Reacting to the December 2 invitation, President Vladimir Putin's spokesman Dmitry Peskov warned that "the continuing expansion of NATO...to the east cannot fail to lead to actions in response from the east -- that is, from Russia."



One possibility is that Moscow could seek to use its influence in Montenegro to try to reverse the government's decision to join the alliance.



That influence includes large Russian financial investments in the Adriatic coast country. Several thousand Russians live in Montenegro, having bought houses and established businesses, particularly in real estate.



At the same time, Moscow could seek to cause social unrest in hopes of toppling the government of Prime Minister Milo Djukanovic. In the run-up to NATO's invitation, pro-Russian opposition groups took to the streets repeatedly to call for his resignation and early elections.

Both of those options may attract Moscow because opinion polls in Montenegro show the population equally divided between those who want to join the alliance and those who oppose it.

WATCH: Residents of the Montenegrin capital, Podgorica, were braced for a negative reaction from Russia after their country was invited to join the NATO military alliance. But they say Montenegro has chosen its own path. (RFE/RL's Balkan Service)

But some observers expect Russia's strongest efforts may be reserved for Serbia, which has close cultural and religious bonds with Moscow -- and whose membership in NATO or the European Union would be seen by the Kremlin as a severe blow.



"[There will be] heightened pressure on Serbia to avoid a similar process of integration of Serbia into existing institutions," Eyal predicts. "I suspect that what you will see is more of a Russian attempt to divide the Balkans between a Western and a so-called Russian sphere of influence."



He says that Russia regards Serbia as a key state because "it is one of the biggest and most powerful countries in the Balkans and because as long as Serbia is not integrated into European structures, it is difficult to see how the Balkans could be secure and develop economically" as part of Europe.



One other possibility for Moscow might be to seek to dissuade some NATO countries from completing the process of inviting Montenegro into the alliance that began December 2. The legislatures of each of NATO's 28 members states must approve that inclusion of Montenegro, meaning the final confirmation of the membership invitation might still be months away.

WATCH: U.S. Secretary of State John Kerry said NATO's invite to Montenegro to join the alliance is only a defensive move. He told Russia it would be a mistake to "react adversely." (Reuters)

Any such eleventh-hour Russian attempt to divide NATO over Montenegro would hope to reawaken earlier reservations among some members regarding whether the small Balkan nation is ready for admission. In the run-up to the December 2 invitation, NATO repeatedly stressed that any offer hinged on Podgorica making further progress on reforms to curb corruption.



Objections from Germany caused a potential invitation to Montenegro to be removed from the agenda of a NATO summit in Wales last year. However, since that summit in September 2014, much appears to have changed regarding how the alliance views the stakes.



"Ultimately, the argument has prevailed that the political messaging that we do not accept the division of Europe is more important than the question of Montenegro," says Eyal. He predicts that the unity NATO showed in offering membership to Podgorica on December 2 will not alter in the months ahead.

Montenegro would be the 29th NATO member state and would follow in the footsteps of other NATO members that may have small armed forces but participate in NATO's operations as part of the alliance's strategy of "burden sharing" among its members.

Montenegro has already explored its likely future role by contributing 25 troops to the NATO-led mission in Afghanistan since 2010 as part of its preparations for joining the alliance.

"Training, advising, and assisting operations is part of what Montenegro has been doing and probably will do when and if it becomes a member state of the alliance," says Beyza Unal, a research fellow in international security at Chatham House in London.

RFE/RL's Balkan Service contributed to this report