The Geopolitics of the United States
The United States needed to expand west to the Pacific coast in order to ensure strategic depth, generate economic activity and development and reduce the presence of any threat on its western front.
The rich agriculture area between the Rockies and Appalachian Mountains has an intricate river system favorable for exports through New Orleans Port via the Mississippi River.
North America can only be threatened from the sea and therefore control of the seas plays an important element of American strategy.
After the Soviet Union fell, the United States had no strategic challenger. The US remains the world’s super power and actively works to prevent the emergence of any potential regional power that could one day challenge it.