On May 11, 1944, the Allies launched Operation Diadem to break through the Axis defences on the Gustav Line. The Gustav Line was the major defence of three that was collectively called the Winter Line. The Winter Line itself was one in a series of defences based on German strategy that relied on the natural features centered on the Apennine Mountains that forms a “spine” along much of the length of Italy.
Successive Allied attacks since January 1944 had failed to breach the Gustav Line. But on May 19, 1944, a concentrated Allied offensive combining U.S. 5th and British 8th Armies finally breached the Gustav Line, forcing German 10th Army to fall back.
Just days later, May 23, U.S. forces that had had also been bottled up for months at Anzio broke out from the beaches and advanced northwest toward Rome instead of attacking northeast to cut off German 10th Army, as planned. As a result, the Allies failed to encircle German 10th Army at the Gustav Line. German 10th Army escaped and, together with German 14th Army from Anzio, soon established new positions in northern Italy. The Allied planning had also placed Rome inside the American sector, and not the British; instead, the latter were tasked to bypass Rome and pursue the retreating Germans.
On May 23, 1944, the Allies broke through the last of the three Winter Line positions, the Senger Line (renamed from the Adolf Hitler Line). Meanwhile, elements of U.S. VI Corps advancing on Rome were stalled by strong German resistance at the Caesar C Line, but exploited a gap and broke through on June 2, 1944. American units entered Rome unopposed on June 4, 1944, which had been vacated by the Germans on orders by Hitler, who balked at another Stalingrad-type attrition battle in the Italian capital. Rome, which had been declared an “open city” and thus undefended, also had been subject to constant Allied air bombardment. The glory attached to capturing Rome, which had predominated throughout the Allied campaign, was upstaged just two days later, June 6, 1944, when the Allies launched the much more strategically important Operation Overlord, beginning with the amphibious landings on Normandy aimed at the re-conquest of France and occupied Western Europe, and ultimately the defeat of Germany.