Donald Trump and Michael Flynn on the campaign trail | George Frey/Getty Images | George Frey/Getty Images Flynn to recommend Trump back NATO membership for Montenegro Move would be in spite of strong opposition from Russia.

National security adviser Michael Flynn will recommend that President Donald Trump support allowing the small Balkan nation of Montenegro to join NATO, POLITICO has learned — despite strong opposition from Russia.

The move will be a major test of the new administration's policy toward Moscow, which considers any further eastward expansion of the Western military alliance a provocation.

Other NATO countries and the U.S. Senate widely support granting membership to the nation of 650,000 people, which once was part of the former Yugoslavia. Montenegro's leaders have accused Russian President Vladimir Putin of fomenting instability inside the country to erode support for joining the alliance — including alleged plots by pro-Russian movements last year to attack the parliament and assassinate the prime minister.

But Flynn, one of Trump's key advisers, "is expected to recommend Montenegro's accession into NATO to Trump in the coming days," a senior administration official said Monday in response to questions.

Trump, who criticized NATO as outdated during the campaign, has praised Putin and vowed to improve relations between Washington and Moscow. Now Montenegro will have an outsize role in revealing how much he is willing to back up the Cold War-era alliance at the expense of his budding relationship with the Russian leader.

“What Russia has done against Montenegro is a unique case,” said Jorge Benitez, a senior fellow and NATO expert at the Atlantic Council, a Washington think tank that supports the expansion. "No NATO candidate country has ever faced such a dire attack or threat in the process of finishing its membership into the alliance.”

Russia has long seen the region as a sphere of influence and has sought to prevent it from falling under the sway of Western powers.

Trump could block the bid under NATO's rules, which require unanimous support from all members. Some supporters of Montenegro's application fear he will oppose extending NATO's defense guarantee to yet another small European country.

“We've defended other nations' borders, while refusing to defend our own,” Trump said in his inauguration speech. “From this day forward, it's going to be only America first.”

But in recent days his administration has taken steps that seem to demonstrate that advisers who push a stronger commitment to NATO and a tougher line against Moscow are having an influence.

Nikki Haley, Trump's ambassador to the United Nations, announced last week that the United States would not lift sanctions against Russia over its 2014 invasion of Ukraine. On Thursday, Ukrainian President Petro Poroshenko called for a referendum on NATO membership, if Russia refuses to pull its forces out of the Crimea peninsula.

Montenegro, which broke away from a state union with Serbia to become independent in 2006, would become the third NATO member in the Balkans, behind Croatia and Albania, which both joined in 2009.

Twenty-three of 28 governments in the alliance have voted in favor of its bid and only the United States, Canada, Spain, Germany and the Netherlands have yet to weigh in. If all NATO members approve Montenegro's membership it will be up to Montenegro's own parliament to ratify the accord.

NATO accession is a highly controversial issue in Montenegro. An opinion poll conducted in December 2016 has only 39.5 percent of Montenegrins in favor of NATO membership and 39.7 against. Other opinion polls have suggested similar margins.

Russia has long seen the region as a sphere of influence and has sought to prevent it from falling under the sway of Western powers. Russia has been accused of bankrolling anti-NATO and anti-European Union political voices throughout the region, including Montenegro’s Democratic Front, a stridently anti-NATO party that won 20 percent of the vote in parliamentary elections last fall.

The party accuses the Montenegrin government of using the NATO issue in order to distract from systemic governmental corruption.

In turn, Montenegrin Foreign Minister Srdjan Darmanović has accused the Democratic Front of being a Russian proxy. “A flood of Kremlin cash went not only to DF and its campaign, but also to media outlets and NGOs that ardently opposed NATO membership,” Darmanović wrote in an article for the Journal of Democracy last month.

In October, Montenegro’s special prosecutor announced that 20 members of a “Russian nationalist” terrorist cell had been arrested on charges of trying to destabilize the country. Exactly what happened is not yet clear, but the apprehended suspects told Montenegrin authorities about an alleged plot to seize the country's parliament building and assassinate Prime Minister Milo Đukanović.

Nebojša Kaluđerović, Montenegro's ambassador to the United States, is adamant that the nation's preparations over the last seven years for NATO eligibility have transformed the country into a strong Western ally.

“It helped bring about our institutions. It helped bring about our democracy. It helps bring stability and security to the whole region,” he said in an interview.

Wide majorities of both parties in the U.S. Senate — whose treaty authority would require its assent — agree.

Senators on both sides of the aisle see Montenegro's bid as a test of resolve against the increasingly belligerent behavior by Russia, which U.S. intelligence agencies have also accused of trying to influence U.S. presidential election by hacking and leaking the emails of Democratic officials and supporters of Hillary Clinton.

The Senate Foreign Relations Committee voted in favor of the treaty with Montenegro on Jan. 11. The panel's chairman, Senator Bob Corker (R-Tenn.), predicts at least 98 senators will vote in favor.

“We’re trying to figure out how to make it happen," Corker told POLITICO. "It will pass 98-2 or 99-1, but getting it on the floor right now is difficult.”

Russia hawks like Senator John McCain (R-Ariz.) worry “a great deal” that Russia will try to destabilize Montenegro before it becomes a full NATO member and has become one of the loudest voices pushing for a full Senate vote as soon as possible.

"I think we should think long and hard if whether or not we are willing to go to war if Montenegro has a skirmish with somebody that surrounds them" — Senator Rand Paul

"We’re doing everything we can to get that up, I promise you.” he said.

“I want to send a clear signal to our friends in Montenegro and to the Russians about how we feel, so I hope we can vote quickly,” added Senator Lindsey Graham (R-S.C.). “The sooner the better.”

Because adding a nation to NATO is a treaty measure, support from two-thirds of senators is required to secure passage. But the Constitution delegates the power to negotiate treaties to the president and Trump could refuse to relay the ratification to NATO, indefinitely stalling the process.

Advocates for delay include Senator Rand Paul (R-Ky.), who has raised concerns about the United States committing to defend another country in which Russia has a strong interest. He blocked a Senate attempt to vote on the treaty in December.

“I think that many are referring to this as a provocation to Russia, and also, I think NATO is too big already," Paul told POLITICO. "I think we should think long and hard if whether or not we are willing to go to war if Montenegro has a skirmish with somebody that surrounds them. Ultimately, joining NATO is not necessarily a benign thing.

“I think there needs to be a debate about how big NATO needs to be,” he added. “We pay for basically the defense of the world. If we let Montenegro in, are they going to provide for their defense or are we going to provide for their defense?”

Kaluđerović insists the government's desire to join the Western alliance should not be interpreted as a sign of aggression against Russia, but rather a desire to be part of the Western world.

“The two pillars of our foreign policy since the day of regaining our independence have been NATO integration and EU integration processes,” he said. “Simply, we belong to this part of the world.

“We are doing that not against anybody, but because we think it is in our favor,” he added.