One reason for the United States' expansion in America during the early 19th century was the idea of manifest destiny, the belief that the U. S. was destined to extend its borders from the Atlantic Ocean westward to the Pacific. This belief grew in part because of the U. S.'s strong stance toward furthering democracy throughout the Americas and partly because adding more territory would prove profitable in the long run.
Another reason for expanding the U. S.'s empire was to protect itself from foreign attack. Although the U. S. has been one of the world's superpowers for nearly a century, it was not so in the 19th century. After gaining its independence from England, the U. S. had to fight a second war with Great Britain (the War of 1812) and a war with Mexico in order to secure its borders. Even Spain was forced to engage the U. S. during the Spanish-American War in 1898. This war helped the U. S. add valued territories outside the continent, including Cuba, Puerto Rico, Guam and The Phillipines--strategic ports in the Caribbean and Pacific that would allow the nation a more expansive world view.