It can be hard for Americans to understand why Russia’s invasion of Ukraine has touched such a raw nerve in Europe. Even the Biden administration did not expect the European response to be so fast and so furious. The widespread expectation, after all, was that sanctions, coordinated between the United States and Europe, would be rolled out slowly. A batch here, another batch a week later, steadily ramping up the pressure on Russia and imposing incremental punishment for its war of aggression. Instead, the sanctions came crashing down all at once. (From what I heard, individual delegations went into the European Council meeting fearing that other delegations would water the sanctions down, so they promised themselves that they would stand firm. On arriving, they discovered that the opposite was true: everyone wanted to hit Russia hard.) In addition to the sanctions, we saw other sea changes on the continent. Germany, which has been understandably wary of militarization, shredded its post-Cold War doctrine over the weekend, agreeing to send lethal aid to Ukraine and to boost its military spending to two percent of G.D.P. The E.U., for the first time in its history, declared it was sending weapons. Switzerland—Switzerland!—which stayed neutral even through the horrors of World War II, decided to impose sanctions on Russia and freeze Russian assets. 

The White House, which had been laying the groundwork for sanctions and trying to get the E.U. to agree—which is typically like herding cats—was taken aback by the unity and swiftness of the response. Why did Europe react the way it did? “Seeing children using metro platforms as bomb shelters stirs the collective memory of World War II,” a U.S. official told me. “It has made them react with anger and emotion, rather than the logic that usually results in the lowest common denominator.”

For America, World War II was Pearl Harbor, island hopping in the Pacific, D-Day, the Battle of the Bulge. As bloody and horrific as those events were, they pale in comparison to what Europe experienced. In six years of war, the continent was leveled. Tens of millions of people were killed in the most novel and horrific ways. Many countries endured the brutality of military occupation. The trauma of that war is still present today in Europe in a way that is foreign to the United States. It is passed down from generation to generation. (The same thing is true of Ukraine and Russia. The Soviet Union lost 27 million people—15 percent of its population—in just four years. Every family lost many, many loved ones, and the trauma of that war is alive and well, thanks in part to Putin’s propaganda machine.) If you could make the images coming out of Ukraine black and white, it might be hard to tell the difference between September 1939 or June 1941. The fact that there is a land war happening again, in Europe, less than a century since the last one, and using a lot of the same language, has been extremely triggering for Europeans (as well as for Russians and Ukrainians). For Europeans, as some of the continent’s officials have told people in the Biden administration, “this is our 9/11.”