Montenegro is set to join the NATO military alliance in early June, after a long political battle between pro-NATO and anti-NATO groups exacerbated by political pressures from abroad.

Even the home stretch proved to be a challenge for Montenegrin Prime Minister Dusko Markovic, who attended the NATO summit in Brussels on Thursday. During a photo op with NATO leaders, he was shoved aside by US President Donald Trump, who was trying to get ahead of the pack. The incident quickly went viral.

The Brussels episode did not go unnoticed in Moscow, with senior Russian lawmaker Alexey Pushkov jabbing that Trump was not "overly polite."

"However, he only showed [Montenegro] its proper place in the alliance," Pushkov tweeted.

The small nation of 625,000 people was part of a federation with Serbia in 1999, when NATO forces targeted the two states to end Belgrade's bloody crackdown in Kosovo. It is also predominately Eastern Orthodox and has traditionally looked to Russia and neighboring Serbia as indispensable allies. While the ruling coalition remains firmly in the pro-NATO camp, the public is almost equally split on the issue of joining the alliance, with some polls indicating a slight advantage for the opponents of the accession process.

Kremlin focused on ex-USSR

Other Russian politicians also expressed disappointment at seeing Montenegro, a fellow Slavic and eastern European nation, openly side with the United States. Last month, the Russian parliament slammed Montenegro's bid to join NATO, saying that "the country's current leadership and its Western sponsors ultimately failed to heed the voice of reason and conscience."

The lawmakers accused "external forces" of trying to undermine "traditions of friendship of the Montenegrins with the Serbs and the Russians." The statement also rehashed the "shameful episodes of NATO's illegal bombing" of Montenegrin territory.

Despite such grumblings from Russia, the Kremlin is not ready to commit significant resources to keep Montenegro out of the US orbit, according to Maxim Samorukov from the Moscow Carnegie Center.

"Montenegro is far from Russia's borders and is already surrounded by NATO countries, and - looking at the big picture - the security situation in Russia will remain mostly unchanged whether Montenegro joins or not," he told DW.

Podgorica commands a minuscule army of some 2,000 troops. Its location on the Adriatic could be strategically important to Russia, as the Russian fleet is always searching for friendly warm-water harbors in the Mediterranean region. However, the Russian leadership is currently focusing its resources on areas closer to home, such as Central Asia, Crimea and Caucasus, Samorukov said.

"There is, of course, this emotional element, but Russian officials are interpreting it as 'liberal, pro-Western elites' taking Montenegro in this direction, towards the US, while the population generally remains pro-Russian," he said, adding that Russia responded similarly to Bulgaria and Slovakia joining NATO in previous years.

'Too relaxed' for attempted coup

The authorities in Montenegro, however, are less convinced of Russia's indifference. Last October, the police arrested a group of Serbian citizens and accused them of trying to overthrow the government in Podgorica and halt the country's bid to join NATO.

The group included an ex-general of Serbian special police forces. Podgorica also believes that two Russian intelligence agents were involved in the plot, possibly with the backing of Russian authorities. Moscow denied involvement.

Montenegrin political scientist Ljubo Filipovic said it is "obvious" that Russia does not want to cooperate in investigating the issue.

"So far, we know that the Montenegrin special prosecutor received no response to his request to travel to Moscow in order to interrogate the two Russian nationals," he told DW. The pair is known to be connected with Russian military intelligence.

Commenting on the alleged coup attempt, Samorukov of the Carnegie Center said that reaction from Podgorica was "too relaxed" for such a serious plot.

The Montenegrin authorities did not close their borders, or start controlling Russian citizens, who are able to enter the state without visas, he added. Also, the plot fell apart in a quite clumsy fashion, making it "difficult to assume that Russia was seriously involved in its organization."

"However, it is possible that there have been individual members of security services working on their own. Maybe they were involved because they were eager to show off their skills, climb the hierarchical ladder and draw attention to themselves, assuming that there really was such a plot," Samorukov said.

Russia to wield pro-Serbian forces

Joining NATO would allow Montenegro to combat such pressures from abroad, said Zlatko Vujovic, head of the Montenegrin Center for Monitoring and Research (CeMI).

Serbia has long maintained a network aimed at influencing Montenegrin politics, including political parties, media outlets, and NGOs, according to Vujovic.

After joining NATO, "Serbian influence will go down dramatically and will continue to weaken, except if Serbia itself changes its foreign policy and perspective on NATO. That appears unlikely at this time," he told DW.

"Russia will try to maintain a large influence, in a way replacing Serbia by using political structures that were once under Belgrade's authority. But even this influence will be smaller than it has been," thanks to NATO's intelligence sharing, Vujovic added.

Montenegro 'crossed the Rubicon'

While Montenegro continues to be divided on NATO membership, the formal entry to the alliance is an important milestone in the country's history, according to Vujovic.

"Even with all these mixed feelings, we can say that Montenegro has crossed the Rubicon and made up its mind when it comes to this east-west dilemma," Vujovic said. "We can say that it has entered the West, conditionally, although we also have to say that it would take a long time to consolidate this pro-Western orientation."