The answer is D.united nations there united with some European countries and that’s how they maintain peace and protection
It could be one of these three:
- American intervention in Japanese conquest towards Manchuria in order to obtain Manchuria's natural resources
- The fact that japanese initiate the attack to Pearl Harbour and drag United States to world war II
- American intervention in Japanese's conquest toward Philippines
The outcome of the Battle of Gettysburg affected morale on both sides by convincing the North that was possible and the South that defeat was inevitable.