WASHINGTON — For years, Ivan Savvidis has been the Kremlin’s man in Greece.

A Greek-Russian billionaire, a former member of the Russian Duma and the owner of a professional Greek soccer team, Mr. Savvidis has moved seamlessly between the sporting worlds of both countries. He has a finger in seemingly every facet of life in Thessaloniki, the Greek port city where he lives, and is a well-known player in the often feuding world of Greek and Russian oligarchs.

All of which has made him of intense interest to American spy agencies.

United States officials say they intercepted communications in June showing that Mr. Savvidis was working as Russia’s conduit to undermine an agreement between Greece and Macedonia that would have paved the way for Macedonia to join the North Atlantic Treaty Organization. Greece has long objected to Macedonia’s entry into NATO. Moscow, which sees the expanding alliance as a major threat on its border, was determined to defeat a referendum on the deal.

In retaliation, American officials made an unusually aggressive move: They turned over the intercepts to the left-leaning Greek government of Prime Minister Alexis Tsipras. The Greek government responded by making a rare break with Moscow, expelling two Russian diplomats from Athens and barring the entry of two more.

American spies and diplomats remain elated. Although a referendum vote last month in Macedonia on the agreement was inconclusive — the Macedonian Parliament must now decide, perhaps as early as this week, whether the country joins the Atlantic alliance — American officials see the exposure of Mr. Savvidis and the expulsion of the diplomats as a rare victory in a catch-up effort against Russian disinformation campaigns in Europe and the United States.

“We’re pushing back and showing that we can play hardball too,” said Christopher R. Hill, a former United States ambassador to Macedonia. “We can tattletale, we can do things that maybe in the past we did not do.”

The secret maneuverings occurred as President Trump traveled to Europe over the summer and publicly embraced President Vladimir V. Putin of Russia and denigrated the alliance — yet another example of the presidency and the career national security establishment seemingly working at cross-purposes. But the Macedonian story begins long before that.

Countering Interference

When American national security officials first saw signs in the summer of 2016 that Russia was trying to interfere in the American presidential election, the State and Defense Departments, working closely with the intelligence agencies, formed what is now called the Russia Influence Group to try to counter the Russian attempts, principally in Europe. That included looking for elections in Europe where Russia might try to intervene.

Late this past spring, they saw that Macedonia was a likely target. On June 12, Macedonian and Greek officials announced that the country would change its name to North Macedonia to end a three-decade dispute with Greece, which saw “Macedonia” as implying territorial aspirations over a Greek region of the same name.

Once the country became North Macedonia, Greece would lift its veto on the country joining NATO. A formal vote to adopt the new name was set for late September.

Moscow was not pleased. At a conference in Athens three days after the agreement, a top Russian diplomat issued a stark warning to the West about the plans to invite Macedonia into the alliance.

“Sure, we will not shoot nuclear bombs,” said Vladimir A. Chizhov, Russia’s ambassador to the European Union. “But,” he added, “there are errors that have consequences.”

Mr. Chizhov’s threat prompted a series of diplomatic cables back to Washington from Geoffrey R. Pyatt, the American ambassador to Greece who was at the same conference, to warn that Moscow was going to try to stymie the upcoming referendum. The State Department, the C.I.A. and the Pentagon’s European Command, based in Stuttgart, Germany, were already on the case, officials said.

By then, opposition to the referendum had already surfaced in Macedonia. Over the summer, there would be hundreds of new websites calling for a boycott of the vote and Facebook posts urging Macedonians to burn their ballots — classic disinformation, Macedonian and American officials said, directed by Russian-backed groups.

The day of the agreement, there were also violent protests outside the Parliament building in Skopje, the Macedonian capital.

Enter Mr. Savvidis. He had been on the radar of American and Greek intelligence agencies for years as a member of Mr. Putin’s party, United Russia, when he was in the Duma, the lower house of Parliament, and had parlayed a fortune first made through a Russian tobacco company into a string of interests throughout northern Greece.

As the owner of the Greek soccer team, PAOK FC in Thessaloniki, he famously stormed the field during a match this year against an Athens team while armed with a holstered handgun and surrounded by bodyguards. The Athens team’s players left the field over concerns for their safety and the match was abandoned.

In June, American officials determined from the intercepts of his communications — email, texts, phone calls or a combination — that he had been paying the protesters.

One senior United States official said American agencies were able to easily collect financial data that put Mr. Savvidis behind payments to citizens and soccer fans to incite violence against the Macedonia referendum.

According to the Organized Crime and Corruption Reporting Project, an investigative reporting organization, Mr. Savvidis paid opponents of the campaign to rename Macedonia at least 300,000 euros, or about $350,000.

Those who received the money included Macedonian politicians, newly established radical nationalist organizations and Vardar club soccer hooligans, who staged the violent protests in front of the Parliament building in Skopje, the project reported.

Mr. Savvidis strongly denied the accusations. “Totally false and highly slanderous,” his holding company said in a statement this past summer.

Relaying the Report

Conveying the American intelligence report to the Greek government in early July fell to Mr. Pyatt, a seasoned American diplomat who has been Washington’s ambassador to Athens for two years, several United States officials said. Mr. Pyatt was well prepared for such as role, having served as the United States ambassador to Ukraine from 2013 to 2016, when Russia seized Crimea and backed separatist forces in eastern Ukraine.

“There’s not a trick the Russians can pull that Pyatt doesn’t know,” Senator Christopher S. Murphy, Democrat of Connecticut and a member of the Foreign Relations Committee, said in an interview. Mr. Murphy said he had spoken to Mr. Pyatt about Russia’s meddling in the Macedonia referendum, but he declined to comment on specifics of their private conversations.

But getting the information to the Greek prime minister was one thing. Getting him to act on it was by no means assured.

The prime minister, Mr. Tsipras, who had refused to join the rest of Europe in backing Britain in blaming Russia for a nerve agent attack on a former spy, was from a political party with a historically cozy relationship with Moscow. Still, the decision to move against Moscow was made easier by the effort Mr. Tsipras had just put into breaking a 30-year logjam with Macedonia, only to see Russia try to spoil it.

Recent American diplomacy helped, officials said. The Obama government stood by Greece during its yearslong financial crisis, advocating for Athens to stay in the eurozone when some European countries initially balked at bailing out Greece. Jacob J. Lew, President Barack Obama’s Treasury secretary, spent so much time on the Greece debt relief package that senior officials joked he had become the Treasury desk officer for Greece. Mr. Obama and Mr. Lew both traveled to Athens during the crisis, as did Vice President Joseph R. Biden Jr., who returned there after leaving office last year.

Mr. Trump hosted Mr. Tsipras last October at the White House, and the two countries are discussing expanding the American military’s presence in Greece, including access for United States ships and aircraft.

“If you look at geography, and you look at current operations in Libya, and you look at current operations in Syria, you look at potential other operations in the eastern Mediterranean, the geography of Greece and the opportunities here are pretty significant,” Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, said during a recent visit to Athens.

Within the American national security establishment, officials have been clearly seeking to nudge Athens away from its relationship with Moscow. “We are cultivating Greece as an anchor of stability in the eastern Med and western Balkans,” A. Wess Mitchell, the assistant secretary of state for European and Eurasian affairs, said in June.

Diplomatic Sparring

On July 11, Mr. Tsipras broke with Moscow. His government expelled the two diplomats and barred two other Russians from entering Greece, accusing them of trying to bribe unidentified officials and foment demonstrations against the Macedonia deal.

“The constant disrespect for Greece must stop,” the Greek Foreign Ministry said in a statement. “No one can or has the right to interfere in Greece’s domestic affairs.”

Moscow later said it was expelling Greek diplomats in retaliation.

Moments after Athens announced it was expelling the Russians, Mr. Pyatt, the American ambassador, triumphantly emailed a number of colleagues and former colleagues at the State Department and the Pentagon. Heather Nauert, the State Department spokeswoman,posted on Twitter after the announcement: “#Greece expelled two Russian officials and barred entry of two others for attempting to interfere in Greek politics. We support Greece defending its sovereignty. #Russia must end its destabilizing behavior.”

The American role in the Greek expulsion also shows how wide a gulf has developed between Mr. Trump — who has publicly questioned NATO while at the same time professing his deep admiration for Mr. Putin — and an American national security establishment that remains rooted to a deeply held belief that Russia’s efforts to meddle in elections both in the United States and among American allies is a geopolitical threat to American security.

That is a thought rarely, if ever, expressed by Mr. Trump. But it is one that a succession of American security officials repeat every day.

“The kind of mischief that Russia has practiced, from Macedonia to the United States,” Defense Secretary Jim Mattis told reporters on Sept. 11, ahead of the Sept. 30 referendum, is “always beyond the pale as far as I’m concerned.” He followed his remarks with a trip to Skopje to “make a specific statement that we stand with the Macedonian people,” he said.

Asked if there was anything the United States was doing to combat Russian efforts to interfere in the referendum, Mr. Mattis said, “Yes.” He declined to go into specifics, but Defense Department officials said that they are also launching information operations, jointly with the governments of Macedonia, Montenegro and Ukraine, to counter Russian disinformation in elections and referendums there.

None of the American behind-the-scene efforts may come to anything if the Macedonian Parliament does not agree to the name change. But Prime Minister Zoran Zaev, who sent the measure to lawmakers this week, said he will call early elections if the measure fails.