As President Vladimir Putin’s atrocities continue to mount, how to end Russia’s war in Ukraine sounds like an arduous question to answer. But according to Putin’s former chief economic adviser, it’s not: Just stop purchasing Putin’s oil, he says. But for real, this time.

If Western countries “would try to implement a real embargo on oil and gas exports from Russia … I would bet that probably within a month or two, Russian military operations in Ukraine, probably will be ceased, will be stopped,” Andrey Illarionov, top adviser and Putin’s G8 representative from 2000 to 2005 told the BBC. “It’s one of the very effective instruments still in the possession of the Western countries.”

Eight years ago, Putin illegally seized Ukraine’s southern region of Crimea in the biggest land-grab in Europe since World War II. The fact that nations overlooked this aggression and proceeded to reward Putin by purchasing more of its oil and gas reflects an abysmal failure to take into account the prospect of more Russian violence. Had Western leaders taken seriously Putin’s territorial ambitions, they would’ve had the foresight to severe ties years ago. Instead, they doubled down on Russian imports, building the $11 billion Nordstream 2 Pipeline to boost deliveries of gas directly from Russia to Germany.

According to the BBC, “Last year, soaring prices meant oil and gas revenues accounted for 36% of Russia’s government spending. Much of that income comes from the European Union, which imports about 40% of its gas and 27% of its oil from Russia.” This money has directly funded and enabled Putin’s atrocities in Ukraine.

Some countries including the United States are finally pushing for a full embargo. But as Politico describes, there are significant holdouts.

Earlier this week an overwhelming majority of European lawmakers demanded an ‘immediate full embargo’ on Russian oil, gas and coal. But while the EU banned the import of Russian coal on Friday, and a number of European governments, including France, Italy, Poland and the Baltics are on board with further sanctions, Germany has repeatedly ruled out an immediate embargo on Russian gas imports, as has Hungary, both citing negative economic consequences for their citizens. Even an end to oil imports from Russia is unlikely before year-end.

A total energy embargo on Russian oil and gas would give the West the autonomy it needs to promote and maintain world peace. In the short term, it could save Ukrainian lives. That Western nations are either unwilling or unable to do this reflects a policy failure of deadly proportions. But as Putin’s former top adviser Illarionov pointed out, it’s not too late to try.