There was a desire for American businesses to open up more markets to sell their goods to other countries. This could be done in the West, and ultimately with counties in the world.

This is political since it says trading with other countries, so then the government contacts other countries with treaties.

Political:

Occupying the middle of the continent from coast to coast would discourage threats from rival powers like England, Spain, or Russia.

This is still talking about governments making treaties with other rival countries, America wanted to lead by example.