U.S. Secretary of State John Kerry, left, with NATO Secretary General Jens Stoltenberg at the alliance headquarters in Brussels, December 2. | AFP/Getty Washington’s Balkan power play America pushes NATO and energy deals to counter Russian influence in a strategic, unsettled region.

BELGRADE — NATO’s invitation to Montenegro this week to join the military alliance was more than a kindness to a small ex-Yugoslav republic. It was another sign of a battle for influence in the Balkans between Russia and the U.S.

And America is the one on the offensive.

In recent weeks high-level U.S. and NATO officials have visited the Croatian, Serb and Montenegrin capitals, seeking to revive stalled efforts to bring the region closer to the West.

Montenegro’s invitation to join the trans-Atlantic alliance is part of that U.S.-led diplomatic initiative. The Kremlin called the move a “provocation.”

Montenegro is the first country in seven years that will come through NATO’s “open door policy” that allows “no third country" a say over its decision, meaning Russia. Behind the scenes at the alliance, the debate was contentious, and Montenegro's invitation far from assured.

Diplomatic struggle

Italy, the Netherlands and Germany have been reluctant to entangle NATO in any direct confrontation with Russia over Ukraine or any further enlargement of the military bloc. French President François Hollande, who along with Germany's Angela Merkel brokered the Minsk ceasefire deal in Ukraine this year, in March said that “France’s position for the moment is to refuse any new membership” offers to NATO.

“Russia is engaged in a massive effort to sway nations. To appeal to them, reach out to them, and fundamentally, tragically, sort of reigniting a new kind of East-West zero sum game that we think is dangerous and unnecessary.” — John Kerry.

In a sign of Berlin's unease, German Foreign Minister Frank-Walter Steinmeier came up with a suggestion to revitalize the moribund NATO-Russia Council, an idea accepted by the rest of the alliance the night before Montenegro's invitation. “I argued very much last night that we try to minimize risks and foster an exchange with Russia,” said Steinmeier.

But the Obama Administration, which hadn't tipped its hand on Montenegro until this fall, and several eastern European countries gave Montenegro the final push in recent weeks, according to diplomats at NATO. They did so for a 9-year-old Balkan nation of 620,000 with close historic ties to Moscow, a standing armed force of just 2,000 and an imperfect track record on the reform of its intelligence and military.

"I would say to Russia and to any other country that worries or thinks about this: This is not focused on them specifically; it’s focused on the potential of defense against anybody or anything that is a threat," said John Kerry, the U.S. secretary of state, at NATO headquarters Wednesday.

After a meeting of NATO ambassadors in the Montenegrin capital of Podgorica in October, the U.S. pushed to bring in the country to encourage continued reform and regional stability, officials said.

“The overall relations with the U.S. in the Balkans have turned more firm and the region came into focus from a wider change on the international scene,” said Igor Tabak, a Zagreb-based defense and security analyst. “The confrontation became visible in the countries where Russia and the U.S. had their interests collide, like Serbia and Montenegro."

NATO’s last two new members were Croatia and Albania, which were invited to join in 2008 and came in the following year.

“Now, with Montenegro, NATO is taking the Balkans piece by piece,” said Stefano Stefanini, former NATO ambassador to Italy who took part in the 2008 enlargement negotiations. “Just imagine the Balkan region on a scale. Every time you move a weight, big or small, you shift the balance — from instability or to stability.”

Washington’s attention revived last year following Russia’s annexation of Crimea and the war in eastern Ukraine.

It prompted Kerry to use Cold War rhetoric in February, naming Serbia, Kosovo, Macedonia, and Montenegro as being the new front line against Russia’s growing influence in Europe.

“Russia is engaged in a massive effort to sway nations,” Kerry told a U.S. Senate subcommittee. “To appeal to them, reach out to them, and fundamentally, tragically, sort of reigniting a new kind of East-West zero sum game that we think is dangerous and unnecessary.”

Fast forward to earlier this week in Belgrade where Kerry met with Serbian Prime Minister Aleksandar Vučić and praised the government’s work on normalizing relations with its breakaway province Kosovo, whose independence Serbia — along with Russia — doesn’t recognize. Vučić visited the U.S. twice this year.

Two weeks ago, Jens Stoltenberg, the NATO secretary general, was in Belgrade, proclaiming a "fresh start" in relations with Serbia, which was bombed over 78 days in 1999 by the U.S. and its allies at the tail end of a civil war in Kosovo. "The aspiration of Montenegro to become a member of NATO is something that is up to Montenegro and NATO to decide," he said.

The NATO campaign in Kosovo left bitter memories in Belgrade, and Serbia is carefully trying to move forward its EU membership application while balancing between East and West and staying militarily neutral.

Shortly after a visit to Washington in September Vučić was in Moscow to meet with Russian President Vladimir Putin. He repeated Serbia’s objection to Western sanctions against Russia over Ukraine. In October last year Serbia hosted a grand military parade in Belgrade with Putin as the guest of honor.

Belgrade’s lack of interest in NATO leaves the U.S. to work on other fronts. Nursing relations with mutual state visits is one way, an active embassy another. A third way is military exercises, said Tabak, pointing out that two months ago Serbian soldiers took part in a military exercise with U.S. troops in Hungary, a NATO member.

“It happens quite often, these regional military exercises with neighboring countries that are NATO members and often it’s with a U.S. component,” Tabak said. ”It’s is a way to strengthen the level of confidence and defense relations."

A tough region

Even as NATO extends an invitation to Montenegro and the country is expected to formally join at the alliance's Warsaw summit in July, public opinion is broadly split.

On one side is the pro-NATO government led by Prime Minister Milo Djukanović, who has ruled Montenegro for a quarter century. He was once a communist, then a nationalist who believed in a Greater Serbia, and starting in the late nineties, a pro-Western proponent of Montenegro's independence. Montenegro split from Serbia in 2006, driving the final nail into the coffin of Yugoslavia.

On the other side of the debate is the pro-Russian and Serbian opposition, which is boycotting the parliament. It staged sometimes violent protests against both Djukanović and NATO this fall.

"The geopolitical role of this region in terms of energy security is now finally being recognized” — Davor Stern, former Croatian minister of energy.

The political crisis in Montenegro — as well as split support for NATO — that festered in recent months led Stoltenberg to make several visits to Podgorica to make sure conditions for joining the club were met.

Diplomats say the country still has a lot of homework to do to fight corruption, which, as in many other Balkan nations, is widespread and rid the Montenegrin intelligence services of Russian influence.

NATO accession is still distant for other Western Balkan countries. The alliance is reluctant to invite members with unresolved territorial disputes, ruling out Kosovo. Bosnia-Herzegovina is struggling with a complicated ethnic-based political system created by the peace accords signed at Dayton 20 years ago, and the Serb-dominated Republika Srpska opposes NATO membership. Macedonia, largely deemed ready for an invitation, is caught in an arcane dispute with Greece, a NATO member, over its name.

Energy piece of security puzzle

Washington is defending its security interests in the Balkans on another strategic front: energy.

Zagreb recently designated a liquid natural gas (LNG) terminal on the island of Krk as critical to Croatia's ambition to be a regional energy hub to lessen the EU’s dependence on Russian gas. The U.S. backs the Krk project as part of a gas corridor running from Azerbaijan via Turkey and Greece up through Albania, Montenegro and Croatia and further north to Europe, a route also also known as the Ionian-Adriatic Pipeline.

“In the future, Croatia can become important as a gateway for energy to Europe,” said Davor Stern, an oil and gas expert and former Croatian minister of economy.

Signalling the American return to the Balkans, U.S. Vice President Joe Biden visited Zagreb in late November, the highest-ranking US official to come to the region since Barack Obama became president in 2009.

The gas supply route through Croatia was raised in talks between Serbia and the U.S. in February. During a meeting in Washington, Biden suggested to Vučić that the Krk terminal could be used to supply Serbia via a pipeline through Hungary.

“It shows that perhaps the geopolitical role of this region in terms of energy security is now finally being recognized,” Stern said.

Serbia is dependent on Russian energy supplies, and a subsidiary of Russia's gas giant Gazprom holds a large stake in one of Serbia's largest oil and gas companies. There was huge disappointment in the country when the Russian South Stream gas pipeline — planned to pass through Serbia — fell through last year.

Hans von der Burchard contributed reporting to this article.

This article was updated to clarify that Croatia and Albania were invited to join NATO in 2008.