The United States' Involvement In World War 2 The United States involvement in World War II was key to the Allies’ success in the war. By the United States providing economic support to their Allies, denying support to the enemy, distributing information, creating key strategies and putting forth so much force in tactical battle the Allies won the war. In essence, if the United States was not involved in the Allied movement against Europe, the European Axis powers would have won the war. The European Axis powers, which consisted of Germany at this time, invaded Poland on September 1, 1939. Poland was a weak force. Their army consisted of