The Mexican Cession of 1848 was taken after the Mexican War.  James K. Polk, an expansionist Democrat, took advantage of a border dispute between Mexico and Texas that came after Texas's war of independence.  Under the Treaty of San Jacinto, both Mexico and Texas claimed the land between the Rio and Nuecces Rivers.  When Polk sent General Zachary Taylor to the disputed region and Mexico predictably fired upon them as trespassers, Polk had the reason to go to war.  The war went disastrously for Mexico due to logistical reasons and its inefficient army. The people of California revolted from Mexico and became the Bear Flag republic.  Under the Treaty of Guadalupe Hidalgo that ended the war in 1848, Mexico ceded all of its land north of the Rio Grande.  In order for America to retain its perceived moral high ground, treaty negotiator Nicholas Trist offered $15 million to the Mexican government.  The Mexican Cession and the ensuing Gadsden Purchase were all part of America's Manifest Destiny, in which many Americans believed that they were destined to rule the North American continent.