The Spanish-American war lasted only a four months, but had far-reaching consequences.
On the foreign affairs front, the war marked the emergence of the U.S. as a world power. The U.S. became a larger player in global politics, entering into treaties and other international agreements. It also marked the end of the Spanish Empire, which had been slowly crumbling since the early 1800’s. The war actually benefitted Spain economically since all the capital from their colonies was eventually returned to them.
The U.S. however began struggling with the idea of becoming an imperialist nation. At first they promised Cuba independence after the war, but the passage of the Platt Amendment kept the island on a very short leash. This is where the naval base at Guantanamo Bay came from.
Theodore Roosevelt returned to the U.S. a war hero. His popularity resulted in a vice-presidential nomination that eventually the presidency.
Strangely enough, the war had a healing effect back home. The photos of southern and northern fighting together helped heal scars left over from the Civil War. It also marked the beginning of an extended period of prosperity in the U.S. that lasted well into the 1920’s.