In June 1944, the U.S. Army participated in Operation Overlord, the Allied invasion of Normandy. After heavy fighting, the Allies broke out of Normandy in July 1944. Efforts to capture the ports in Brittany proved difficult, but Allied forces quickly liberated Belgium and much of France. U.S. forces participated in the invasion of southern France (Operation Dragoon) in August 1944, and the airborne assault on the Netherlands (Operation Market Garden) in September, but logistical difficulties and German resistance slowed U.S. assaults on the Siegfried Line and in Lorraine in late 1944. In December 1944, the U.S. Army was surprised by the German Ardennes offensive (the Battle of the Bulge), which was defeated only after costly fighting that strained the U.S. Army's manpower to its limits. In February 1945, operations began that drove the Germans from the Rhineland, and the Rhine was crossed in March 1945. In April 1945, U.S. Army forces entered the heartland of Germany. Operations continued until the war in Europe ended on 8 May 1945 with the signing of unconditional surrender the previous day.