Without Russian gas, Europe’s economy will tip into a recession, which explains why leaders resisted gas sanctions earlier in the Ukraine war. But that vacillation has increased Mr. Putin’s leverage. Europe’s gas purchases, at higher global prices, have furnished the Kremlin with a windfall. The International Energy Agency estimates that the additional revenue Russia has earned from Europe in the past five months compared to a normal year is more than three times what Russia would earn from Europe in a normal winter.

What now for Europe? At best, politicians shaken by this week’s scare will ramp up their planning for realistic alternatives. Those include arranging liquefied natural gas supplies from elsewhere, or in Germany’s case keeping its three last nuclear-power reactors online.

The danger is that politicians instead will increase pressure on Kyiv to negotiate a settlement on the Kremlin’s terms. French President Emmanuel Macron has seemed open to this, German Chancellor Olaf Scholz wouldn’t require much persuading, and Italian Prime Minister Mario Draghi might not be in power long enough to hold a firm line.