Hitler retreated to his "Führerbunker" in Berlin on 16 January 1945. It was clear to the Nazi leadership that the battle for Berlin would be the final battle of the war in Europe. Some 325,000 soldiers of Germany's Army Group B were surrounded and captured in the Ruhr Pocket on 18 April, leaving the path open for U.S. forces to reach Berlin. By 11 April the Americans crossed the Elbe, to the west of the city. On 16 April, Soviet forces to the east crossed the Oder and commenced the battle for the Seelow Heights, the last major defensive line protecting Berlin on that side. By 19 April, the Germans were in full retreat from Seelow Heights, leaving no front line. Berlin was bombarded by Soviet artillery for the first time on 20 April, which was also Hitler's birthday. By the evening of 21 April, Red Army tanks reached the outskirts of the city.