Montenegro has formally joined NATO, with U.S. and Montenegrin officials sending subtle messages to Russia and the alliance’s chief trying to allay concerns about the U.S. commitment to the alliance.

Montenegrin Foreign Minister Srdjan Darmanovic handed over the Balkan nation’s accession papers in a ceremony at the State Department’s ornate Treaty Room on June 5, making the country the 29th member of the North Atlantic alliance.

The State Department’s third-highest official, Undersecretary of State for Political Affairs Tom Shannon, said that Montenegro had asserted "its sovereign right to choose its own alliances even in the face of concerted foreign pressure."

Prime Minister Dusko Markovic, who also attended the event in Washington, alluded to his country’s past plight at the hands of larger powers.

"It is a historic event for a country and a nation which endured enormous sacrifices in the 19th and 20th centuries in order to defend their right to a free life, the right to decide our own future, recognized by the world under our own name, and with our own national symbols," Markovic said.

"We are celebrating today the fact that it will never happen again that someone else decides instead of us and our state behind our back, as was the case in the past,” he added.

Montenegrin membership comes at a time when the alliance is showing strains among its members, particularly in the face of mixed messages from U.S. President Donald Trump.

During his recent trip to Europe, Trump scolded allies for not meeting pledges for defense spending, and repeated his position that Washington was shouldering too much of the burden.

What particularly worried allies, however, was Trump’s lack of explicit endorsement of the alliance’s most important clause, known as Article Five. That clause is a “one-for-all, all-for-one” provision that obligates all members to come to the defense of any other member who is attacked.

It has only been invoked once in the alliance’s nearly 70-year history: after the September 11, 2001, terrorist attacks in the United States.

Politico reported on June 5 that the omission of Article Five from Trump’s speech caught his top national security officials off-guard, potentially signaling confusion or internal clash among White House officials.

Trump also caused a minor stir in Brussels last month when he appeared to push Markovic aside during a group photo for NATO leaders.

NATO Secretary-General Jens Stoltenberg alluded to the ongoing disputes between the Trump administration and its European allies. German Chancellor Angela Merkel, in particular, has signaled that any U.S. pull back meant that Europe must become more united.

"We are an alliance of democracies and we have, at times, different political perspectives. But together, we rise above those differences and unite around a common purpose: to stand with each other, to protect each other, and -- if necessary -- to fight to defend each other," he said during the ceremony.

For Montenegro, whose population is mostly Orthodox Christian, alliance membership offers protection from Russia, who experts believe has stepped up efforts to destabilize nations in the Western Balkans.

Montenegrin officials have charged 14 people in connection with an alleged Russia-backed plot to take over parliament during the October 2016 parliamentary elections and assassinate then-Prime Minister Milo Djukanovic.

In February, Markovic called out Russia and its allies within the country, warning of unrest and saying they should stop trying to interfere.

Russia has made no secret of its opposition to NATO membership. Moscow has said that Podgorica's NATO course runs counter to hundreds of years of "fraternal relations" between the two mostly Slavic, Orthodox Christian nations.

Russia wields considerable economic power, as it is the largest investor and an estimated 7,000 Russian nationals permanently reside in the nation of about 620,000.

Russians also own about 40 percent of the country's desirable Adriatic Sea coast and Russian tourists account for as much as a third of overnight visits in Montenegro.

"The anti-Russian hysteria that continues in Montenegro has only caused regret," the Foreign Ministry said in a statement released ahead of the ceremony. "In the light of the hostile line of Montenegrin authorities, the Russian side reserves the right to take retaliatory measures on the basis of reciprocity. In politics, as in physics, every action generates a reaction."

Still, analysts said it is unlikely Moscow will take any real action over Montenegro’s accession.

“The Kremlin isn't ready to commit significant resources to keep Montenegro out of the U.S. orbit,” said Maksim Samorukov, an analyst at the Moscow Carnegie Center.

RFE/RL’s Balkan Service contributed to this report.