The 19th century belief that the United-states would inevitably expand westward to the pacific ocean and into Mexican territory.
Herein, what was the agreement made in the Fort Laramie Treaty?
What was the agreement made in the Fort Laramie Treaty? Gave Native Americans control of much of the Great Plains, and, in return, agreed not to attack settlers as they moved west.
Accordingly, who added 1.2 million square miles of territory to the United States? The Mexican War lasted less than two years and tremendously increased the size of the United States by adding 1.2 million square miles of territory including the present-day states of California, Nevada, Utah and large parts of Colorado, Arizona and New Mexico.