Complex sentence: Germany fought the war on multiple fronts through 1942--1944, however following the Allied invasion of Normandy (June 1944), the German army was pushed back on all fronts until the final collapse in May 1945.
Split into simpler sentences: Germany fought the war on multiple fronts through 1942--1944. Following the Allied invasion of Normandy (June 1944), the German army was pushed back on all fronts until the final collapse in May 1945.