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.
I believe it is a means to harvest the area of all timber, which can be beneficial for economic reasons and the ecosystem
Nobles and knights were obliged to protect those who lived on the manor and provide personal and military service to the king.