With the annexation of Texas in early 1845, war with Mexico became all but inevitable. President James K. Polk hoped to settle matters peacefully but was determined to have his way by war if necessary. In November 1845 he sent John Slidell to Mexico with an offer of $5,000,000 for the purchase of New Mexico and $25,000,000 for California. The offer was refused. On May 9, 1846, word reached Washington, D.C. that American troops had been attacked by Mexican forces on April 4. Polk asked Congress, and was granted, a declaration of war. Congress authorized the president to call for 50,000 volunteers and appropriated $10,000,000. Congress and the nation, however, were far from united on the idea of waging war. Southerners favored war as likely to extend slave territory, while notherners opposed the war for the same reason.

President James K. Polk ordered U.S. troops into the region in southern Texas disputed with Mexico, between roughly parallel Nueces and Rio Grande rivers. The commanding general was Zachary Taylor.

1846- Battle of Palo Alto, the first important engagement of the Mexican War, was faught, with the Mexicans on the losing side.

1846- Battle of Resaca de la Palma, U.S. forces under General Taylor forced the Mexican army back across the Rio Grande.

1846- U.S. forces crossed the Rio Grande, led by General Zachary Taylor. The Americans occupied Matamoros.

1846- Monterrey, Mexico, was captured by U.S. forces under General Zachary Taylor after a four day engagement that made "Old Rough and Ready" Taylor, a Whig, into a national hero. His relations with President Polk, a Democrat, cooled subsequently.

1847- Battle of Buena Vista, U.S. forces under General Zachary Taylor defeated the Mexicans under General Antonio Lopez de Santa Anna.

1847- U.S. forces under General Winfield Scott landed near Vera Cruz, Mexico. Some 10,000 troops landed in the Western Hemisphere, in what was the first large=scale amphibious operation in U.S. history. Scott began a siege of Vera Cruz on March 22. The fortress fell on March 27 and was occupied two days later. On April 8 Scott moved toward Mexico City.

1847- To negotiate peace with Mexico, President James K. Polk appointed as his special agent Nicholas P. Trist, a State Department veteran.

1847- At Cerro Gordo General Winfield Scott, marching on Mexico City, met and defeated a Mexican force of about 13,000.

1847- Peace negotiations with Mexico were initiated through the British minister, Charles Bankhead.

1847- At Churubusco General Winfield Scott defeated a Mexican army of 20,000.

1847- At the Battle of Molino del Rey U.S. forces under General Winfield Scott defeated an estimated 12,000 Mexicans.

1848- The Treaty of Guadalupe Hidalgo, signed by the U.S. and Mexico on February 2, 1848, formally ended the Mexican War (1846-1848). By the terms Mexico recobnized Texas as part of the U.S. and ceded to the U.S. over 500,000 square miles of territory, including all of the future states of California, Nevada, and Utah, almost all of New Mexico and Arizona, and parts of Colorado and Wyoming. In return the U.S. agreed to pay Mexico $15,000,000 and to assume the claims of U.S. citizens against Mexico, amounting to $3,250,000. The U.S. became an enormous continental republic, but the acquisition of the new territory aggravated the dispute between slavery and antislavery forces. The war resulted in 1721 dead and 4102 wounded. In addition, some 11,155 Americans died of disease as a result of the war. The total cost of the war was estimated at $97,500,000.