Winston Churchill once quipped that George C. Marshall, the famed American general who served as Army chief of staff during World War II, was part of the “stupidest strategic team ever seen.” The cause of the British prime minister’s ire? Operation Anvil (later renamed Dragoon), the Allied invasion of southern France that began 75 years ago Thursday. Today that operation, which historians consider one of the most successful amphibious assaults in history, is largely forgotten, overshadowed by the D-Day invasion, which took place a few months earlier and on the opposite side of France.

But Anvil is worth remembering not just for its sophisticated planning. While the British adamantly opposed the operation, the Soviets just as adamantly supported it. When President Roosevelt seemed to side with the Russian dictator, Joseph Stalin, over Churchill, the trans-Atlantic alliance hit its low point.

The idea behind Operation Anvil emerged in mid-1943 as one of many potential operations against Continental Europe. It solidified in late November of that year, when Churchill, Roosevelt and Stalin met for the first time in Tehran, to determine the war’s strategic course in 1944. They agreed on simultaneous offensives in the summer, with Operation Bagration, an enormous Soviet operation, along the Eastern Front and Operation Overlord, the official code name for D-Day, in northern France.

But Stalin, who had been demanding that Britain and the United States open a Western Front against the Germans for months, believed that an invasion through southern France was the best way to augment Overlord. American planners responded with Anvil, which would cut up from the French Mediterranean coast along the Rhone Valley, toward southern Germany. The Americans liked the idea because it would open a new supply route into Europe, and because it would catch the Germans in a classic military pincer movement.