The correct answer is: "the entrance of the US on the war allowed to defeat Nazi Germany in the Western Theatre".
Since the start of WWII in 1939, until the direct involvement of the US in 1942, Hitler managed to conquer most territories in Western Europe: Belgium, the Netherlands and most of France (except Vichy regime in the South), and the nazis kept on bombing the UK, that was more difficult to conquer due to the natural barrier of the sea.
The arrival of the US forces managed to stop that dominance of the nazis in the Western front and, after some military operations, specially the D-day Normandy landings in 1944, the Allied powers leaded by the US managed to conquer back and to free Western Europe.
