Mexico held the southwest territories and the US wanted them. The US annexed the Republic of Texas in 1845 along with Florida. President Polk sought to expand the nation to the Pacific coast.
John C. Fremont led a force into California in the winter of 1845-46. Gen. Zachary Taylor had already been sent to safeguard the Texas border. The US declared war in May 1846.
Exiled former president Santa Anna told the Mexican government that he wished a military command, but had no aspirations on the presidency. He had also been negotiating with the Polk. If he gained the presidency, he would negotiate with the US for the territory it desired.
Santa Anna returned from Cuba to Mexico City, declared himself president and led the army against the US.
Northern and Southern Whigs opposed the war, fearing, in part, the expansion of slavery. Southern Democrats were in support of the conflict.
Zachary Taylor led the "American flying cavalry" against Palo Alto. Gen. Stephen Kearney claimed New Mexico Territory for the US and appointed himself military governor. Kearney soon left for California, leaving Col. Sterling Price to ward off uprisings around Santa Fe. Fremont led the California campaign with the help of a naval blockade. Kearney also led a force into Los Angeles.
Santa Anna met Taylor at Buena Vista, nearly besting the Americans, who valiantly held on to the mountain pass. Word came of trouble in Mexico City and Santa Anna was forced to withdraw.
Taylor became a military hero with his victory at Palo Alto and Winfield Scott with his Veracruz victory. Scott also led the troops that secured Mexico City. Santa Anna was finally defeated at Huamantla and forced to relinquish his command.
The Treaty of Guadalupe Hidalgo ended the war and set the Mexican border at the Rio Grande River, giving the US the former Mexican lands to the north, for which the US paid approximately $13 million. The US also assumed a Mexican debt of over $3 million.
Family connections: None that I am aware of.