The Mexican Cession was the result of a war caused by the Americans attempting to gain Mexico's land. The Treaty of Guadalupe Hidalgo marked the peace. It was in 1848 that the United States gained 500,000 square miles, increasing the size of the U.S by 25%. The Senate ratified the treaty in March of 1848. The states of California, Nevada, Colorado, New Mexico, Utah, most of Arizona were formed from this land.