There were at least three reasons for this.
- Economics. Many people felt that America needed new sources of raw materials and new markets where American companies could sell goods.
- Military power. This idea is connected to Alfred Thayer Mahan. He argued that naval power was the key to world power. A strong navy needed bases around the world to refuel and resupply and from which to project power.
- The "White Man's Burden." Many Americans believed that the US was superior to other nations and that it had an obligation and a right to spread its civilization to other countries even if it had to do so by force. It was essentially going to be colonizing people for their own good.
It was for these reasons that the US decided to expand beyond the continental United States in the late 1800s.