FOR YEARS, Western governments have chafed at the presence of a repressive dictatorship in the center of Europe — the former Soviet republic of Belarus, ruled with a mixture of brutality and megalomania by Alexander Lukashenko. Now a spectacular series of missteps by the strongman has put him in perhaps the tightest fix of his 17 years in power — and given the United States and its European allies an opportunity they should not fail to exploit.

Mr. Lukashenko’s troubles began last December when, after promising to hold a fair presidential election in exchange for a $3.6 billion Western aid package, he blatantly rigged the vote. When tens of thousands of Belarusans turned up in the center of Minsk to protest the fraud, he sent his police to attack them. Hundreds were arrested, including seven of the nine candidates who ran against him.

In recent weeks, a series of show trials have been staged against dozens of opposition leaders, including five of the presidential candidates. Twenty two have been given prison terms so far, including Andrei Sannikov, Mr. Lukashenko’s principal opponent, who was sentenced to five years in prison last Saturday. Claiming that he has crushed a Western-backed “fifth column,” Mr. Lukashenko has further justified the crackdown by citing a mysterious terrorist bombing in the Minsk subway system last month that killed 14 people and injured 200 — an attack that some in the opposition believe the regime itself may have orchestrated.

Oddly enough, Mr. Lukashenko himself spelled out his vulnerability in an interview with The Post’s Lally Weymouth in March. “The stupidity,” he said, “is that Belarus and Lukashenko do not have the resources to be a dictator.” In fact, having promised huge pay increases to state employees before the election, the regime is now watching the economy implode because of Belarus’s inability to obtain foreign financing. The value of the currency has plunged by 50 percent in the past week; hundreds of businesses are closing; and shortages are spreading.

Russia, which usually bails Belarus out, is playing hardball. It has withheld a loan package and even condemned the political trials. This is not because Moscow has discovered human rights; rather, Prime Minister Vladi­mir Putin, who has long despised Mr. Lukashenko, is trying to force him to sell off large state companies to Russian investors and cede Belarus’s political independence to the Kremlin.

Mr. Lukashenko has tried to resist Mr. Putin in the past by playing him off against the West. But Western governments should no longer succumb to that tactic. The United States and the European Union have already imposed sanctions on more than 170 Belarusan officials since the post-election violence, and they should respond to the trial verdicts with still tougher measures. In particular, European foreign ministers should move at their meeting on Monday to penalize the big state companies — including the oil, potash and arms export firms — on which the dictatorship rests. They should let Mr. Putin know that swallowing Belarus and its assets will prejudice Russia’s own proposals for a strategic partnership with the European Union.

For its part, the Obama administration should make Belarus a test of the “reset” of relations with Russia. Moscow and Washington ought to be able to agree on a common message to Mr. Lukashenko: free the political prisoners and accept genuine political and economic reforms, or face bankruptcy and isolation.