Russian President Vladimir Putin famously miscalculated by sending an unprepared army to fight a four-day war in Ukraine that’s now in its fourth month. But Putin got one crucial thing right: the value of oil as a shield to protect Russia’s economy and keep his ruthless war going.

The United States, Europe and many other countries have layered withering sanctions on Russia, cutting many of its banks and much of its economy off from the rest of the world. Those are now causing widespread shortages of imported goods and other problems, resulting in hefty inflation and a nationwide decline in living standards for ordinary Russians.

But sanctions have not stanched Russia’s revenue from oil sales, the single largest source of government funding. In fact, the spike in oil prices caused in part by Putin’s own war led to record-high energy revenue for Russia in March and April—when most of the sanctions were already in effect. In the first four months of 2022, Russia earned half of its planned energy revenue for the year, putting it on a path to exceed its own targets while prosecuting the biggest European war since the Allies defeated Hitler in 1945.

That energy revenue is a vital shock absorber that lets Putin prolong his war, offset some of the pain Russian citizens feel and maintain his iron grasp on power.

“Russian imports are collapsing, but Russian exports, because of the high cost of energy, are actually doing very, very well,” Iikka Korhonen of the Bank of Finland said during a May 25 online forum sponsored by the Peterson Institute for International Studies.“If you want to do something to Russia’s fiscal revenues, trying to have an effect on energy exports is of utmost importance.”

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Europe is now trying to do exactly that. On May 31, the European Union reached a deal to phase in a partial ban on Russian oil and oil products. The aim is to ban up to 90% of the oil Europe buys from Russia by the end of the year, with an exception for oil sent by pipeline to a few landlocked countries that can’t get oil from other sources by ship. A ban on refined Russian oil products will kick in next year, assuming there’s no resolution to the war. A complete boycott starting immediately, as the United States and United Kingdom have imposed, would obviously be more effective, yet the European bancould still thin Putin’s main source of cash, if it holds.