Mostly due to the ideas of Manifest Destiny. They started going westwards with the desire to span the United States from coast to coast, and then they started going worldwide. The idea behind it is that the United States were destined to help the world become free and democratic like they were so they started helping people worldwide in rebellions against colonialism or in similar affairs.
D massive amount of territory lost= Rhineland
They all killed millions of people
North towards the middle east, before moving east towards Thailand, before going south
