The German military was mostly unstoppable since the start of World War 2 in 1939. The German military was using the blitzkrieg tactic and had successfully invaded France, Belgium, Holland, Norway and Demark. They were predominantly undefeated until the allies invaded Normandy on D Day.
The America military was critical to D day. In December 1941 Japan attacked pearl harbour, which lead to the United States becoming involved in World War 2. When the United States join the allies, a strategy was agreed that all the allies would joint up to defeat Germany in Europe before defeating Japan.
At the Allied Quebec conference in 1943 the allies outlined the plan for D-day