Manifest Destiny was the belief that the United States had an obligation to expand from coast to coast and an obligation to increase the domain over which democracy and liberty held sway. That was a nice way of saying we would eventually rule most, if not all, of North America. We believed our system of government and our economic system was better than others and we owed other peoples (like Canada and Mexico) to make them embrace our system. Business favored the idea because it meant two things of importance to our economic system. Raw materials would become available to the industries and factories, and the people in the new areas would become consumers and open up a large economic market place for out nation.
