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.
The upheaval was caused by widespread discontent with the French monarchy and the poor economic policies of King Louis XVI, who met his death by guillotine, as did his wife Marie Antoinette
