It was to be a hugely ambitious project on the frigid Gulf of Ob, in Russia’s Far North, a steppingstone in Moscow’s rising ambitions to be a power in liquefied natural gas much as it is in oil and gas delivered by pipeline.

When President Vladimir V. Putin of Russia launched his war on Ukraine, the $21 billion project known as Arctic LNG 2 was well underway with dozens of wells drilled, an airport built and most of the equipment ordered.

Now, though, European Union sanctions that prohibit the sale of gas liquefaction equipment to Russia have thrown the giant complex into doubt. The sanctions mean, at best, that just one of three planned liquefaction facilities at Arctic LNG is likely to be completed anytime soon, analysts say.

The project’s key international backer, TotalEnergies, recently wrote off its $4.1 billion investment. It is “difficult to believe that it can be built with the sanctions,” Patrick Pouyanne, the TotalEnergies chief executive, told analysts in late April.

The problems go far beyond L.N.G.

In coming years Russia is likely to be forced into retreat across a wide spectrum of energy. Future growth for its oil and gas exports — for decades the backbone of the country’s economy — is now deeply uncertain. The shock waves from Ukraine even extend into nuclear power, where Finland recently shelved a deal for Russia to build an estimated 7 billion euro ($7.4 billion) reactor.

“Russia is going to be a greatly diminished international player, there is no question about it,” said Matt Sagers, vice president and head of Russian and Caspian energy at S & P Global, a financial services firm.

Russia will probably find markets for at least some of its oil and gas; in April Mr. Putin said that while the country may lose traditional buyers, it would find more at home and abroad. But it may gradually lose influence in the industry, becoming a pariah to former international partners like the major international oil companies.

Some analysts also say it is hard to see how Moscow can remain a co-chair, with Saudi Arabia, of the oil producers’ organization known as OPEC Plus. So far, though, Saudi officials and others are sticking with Russia with an eye toward preserving cohesion in the group for a distant future date when the world is oversupplied with oil rather than worried about shortages.