As elsewhere in Europe, Mr. Putin courted Bulgaria’s pro-Russian far right party, which promoted Russian interests by helping to beat back plans to explore for Bulgarian shale gas. Seeking to nail down support for South Stream, a member of the Russian Parliament, whose card identified him as a special emissary of Mr. Putin, even offered what one former Energy Ministry official understood to be a tacit bribe.

“It’s not just South Stream, and it’s not just Bulgaria,” said the Bulgarian official, Bojan Stoyanov, who was deputy energy minister in 2013. “The Russians are promoting behavior where people are willing to dig the hole of a volcano in the middle of their country and not worry where the lava goes as long as they get paid.”

Putin’s Favorite Musicians

Vladimir Putin had a problem.

By 2011, his plan to build South Stream, estimated to cost more than $40 billion and four years in the making, had hit a stumbling block in Bulgaria, where the pipeline would make landfall after traversing the bed of the Black Sea. Geological surveys suggested that Bulgaria could be sitting atop an underground ocean of natural gas, enough to be self-sufficient for years, enough to eclipse the advantages of South Stream.

Bulgaria, once a staunch Soviet ally, and Russia share similarities in language, religion and culture, and Bulgaria still celebrates as a national holiday its 1878 liberation from the Ottoman Empire by the troops of the czar. Bulgarian leaders supported South Stream, declaring that the pipeline would provide not just transit fees but energy security: The country receives 90 percent of its gas from Russia, along a route through Ukraine that has left it vulnerable to periodic pricing disputes between Moscow and Kiev. In the winter of 2009, Bulgarians were left shivering for two weeks when Russia shut off the gas to teach Ukraine a lesson.

But in the middle of 2011, Bulgaria’s prime minister, Boiko Borisov, also granted Chevron a permit to explore for shale gas. Almost immediately, a well-organized campaign emerged to kill shale exploration before it began, fueled in part by loyalists for Ataka, one of several far-right parties that Mr. Putin has cultivated in Europe. Parties like Jobbik in Hungary and the Northern League in Italy view Mr. Putin as a bulwark against a tide of Western liberalism. But the appeal is more than ideological. In France, the leader of the far-right National Front, Marine Le Pen, recently acknowledged that her party had received a loan for 9 million euros, or about $11 million, from a Kremlin-linked bank.

In Bulgaria, Volen Siderov, the chairman of Ataka, said his party received no direct funds from Moscow.