First of all, the Louisiana Purchase from Napoleon gave the US a vast amount of land (doubled the size of the United States at the time).
Second was that,
one of the causes of the War of 1812 was that Britain desired to begin an Indian State in the area south of the great lakes, in Ohio, Michigan, and Indiana.
The United States sought to entitlement this area, and brawled and won a war for it
hope it helps