The Cold War started to dominate national affairs and the United States won. No fighting was involved, and some people believe the war hasn't yet ended.
One cause of the war could have been the Soviets, who didn't follow their agreements to allow the people of Eastern Europe to determine their own futures by imposing a centralized government that didn't tolerate differing opinions on territories unlucky enough to fall behind the Iron Curtain. The Iron Curtain is a barrier made by the Soviet Union after World War II to seal off itself and its eastern and central European allies from open contact with the West and other non-communist areas.
Another cause could have been the Americans, who ignored the Soviets' security concerns, pushed relentlessly to expand their own international influence and promote dominance, or trying to intimidate the world with the atomic bomb.