As a result of winning what war did the us become an empire? what colonies did it gain?

An Empire, Well I wouldn't call us an Empire, but after the Spanish-American War in 1898, The United States DEFINITELY became an Imperial power.

The United States had gained Guam, Philippines, Hawaii, and Puerto Rico. Although they did not own Cuba because the Teller Amendment forbid America to do so, the sneaky Platt Amendment had forced Cuba to sell lands to America... Of these lands are Guantanamo Bay, which, although the Platt Amendment was overturned during Roosevelts term, still stays a US possession today.