Yet there are reminders of the carnage. People occasionally find shards of shrapnel in the trunks of felled trees. Farmers often must call in a munitions squad to remove undetonated explosives from their fields. Overlooking Château-Thierry is the monument commemorating the Americans who fought at the Marne. Outside the city are two American military cemeteries — the Aisne-Marne and the Oise-Aisne, both maintained by the United States government — which together have rows of white tombstones for more than 9,000 men and women who were sent off to a distant war and never came back.

The United States entered World War I in 1917 with an untested force whose leader, General John J. Pershing, nicknamed “Black Jack,” was opposed to fighting under a French command. He relented when the Germans began major offensives in the spring of 1918 to try to win the war before the Allies could gather strength from the arriving American forces. They retook an important ridge position at the Chemin des Dames, and then blitzed another 40 or 50 miles to the banks of the Marne.

Four years earlier, with the Germans sweeping toward the capital, the French stopped them in the First Battle of the Marne as taxi drivers from Paris ferried reinforcements to the front. That victory led to four years of bloody stalemate in the trenches, but in 1918 the Germans were again on the banks of the river. Historians now doubt that the Germans intended to make an immediate frontal blitz on Paris, but a German victory at the Marne could have been devastating for the Allies.