Montenegro’s membership will pull the nation closer toward Europe's political and economic structures, but will also sharply deepen Moscow's distrust of the alliance, whose eastward expansion has long angered Russia.
As with neighboring Serbia, Montenegro shares linguistic and cultural roots with Russia, and Russians, including the Kremlin-connected billionaire Oleg Deripaska, have made substantial investments in the country in recent years.
Even before the Senate vote, Moscow signaled deep opposition to Montenegro’s efforts. A coup attempt last year, which some Montenegrin lawmakers blamed on Moscow, was seen as a possible effort by Moscow to undermine the NATO push.
U.S. support for Montenegro’s bid has been mostly strong, though some observers have speculated that President Donald Trump’s conciliatory rhetoric toward Russia might result in weaker support for NATO expansion.
However, ahead of the vote, Secretary of State Rex Tillerson wrote to Senate leaders, saying approval should come ahead of a key NATO summit scheduled for May.
That would include ex-Soviet republics who have faced Russia's military aggression: either outright invasion, in the case of Georgia, or a separatist insurgency backed by Moscow, in the case of Ukraine.
"If we were to lose this, it would set back many of the other countries and peoples, particularly in Eastern Europe, who are looking forward to, and have their eyes set on the West," Scaparrotti said.
Russia's strongest ally in the Balkans, Serbia, has also moved gradually toward closer integration with the European Union, though not with NATO.
Moscow appeared to send a signal to both Serbia and others that it remained fully engaged in influencing Balkans politics by hosting Serbian Prime Minister Aleksandar Vucic in Moscow on March 27, a week before Belgrade holds presidential elections.
Vucic is the favorite in the April 2 vote to succeed President Tomislav Nikolic, who has decided not to seek a second term. A populist, Vucic has said he wants to accomplish bringing Serbia closer into the EU but also improve ties with Moscow.
Vucic also confirmed that Russia would provide "as a gift" six MiG-29 fighter jets to Serbia in the coming weeks.