Why does President Polk think the U. S. should extend its borders?

Territorial expansion of the United States on the Pacific coast was foremost in the minds of President Polk and his associates in their whole conduct of the war. The major consequence of the war was the Mexican Cession of the territories of Alta California and Santa Fe de Nuevo México to the United States under the Treaty of Guadalupe Hidalgo. In addition, Mexico accepted the loss of Texas and the Rio Grande boundary.