The Spanish-American War (1898-1900).  By the close of the nineteenth century, the United States stretched across the continent, and the Nation began to expand its power abroad. Under the prodding of naval officers, such as Alfred Thayer Mahan, and civilians such as Teddy Roosevelt, America began to build a modern steel navy and establish naval stations both at home and around the world. At this time, Spain's possessions included Cuba and Puerto Rico in the Atlantic Ocean, and the Philippines and Guam in the Pacific Ocean.

After the inauguration of President William McKinley in 1897, American attention increasingly focused on the island of Cuba, just ninety miles from Florida. Spain was trying to stop a rebellion in Cuba using harsh methods. Continuing newspaper reports of Spanish atrocities and the sinking of the American battleship, USS Maine, in Havana Harbor caused American feelings toward Spain to turn hostile. At the same time, the rebellious Cubans were demanding concessions from their Spanish overseers. When these demands were not met, the United States Congress declared war on Spain in April, 1898.

The Spanish-American War showed that the U.S. Navy was much better prepared to fight than the U.S. Army. Within a short time, naval victories in the Philippines and Cuba, coupled with the landing of the U.S. Army in Cuba, led to the end of the war. A peace treaty between Spain and the United States was signed in December, 1898. Cuba was granted its independence, but the United States decided to keep the Philippines, Guam, and Puerto Rico for a few years. America now had an overseas empire.