Complex sentence: It was initially filmed before the December 1941 attack on Pearl Harbor brought the USA into the war, but following the attack, the scene was filmed again to reflect the tough, new spirit of a nation at war.
Split into simpler sentences: It was initially filmed before the December 1941 attack on Pearl Harbor brought the United States into the war. Following the attack, the scene was filmed again to reflect the tough, new spirit of a nation at war.