Timeline of Mexican-American War

March 4, 1845: James K. Polk, who advocated for the annexation of Texas and believed in the “Manifest Destiny” of the U.S. assumes Presidency in the United States.
June 16, 1845: The U.S. officially annexes Texas, though Mexico still does not recognize its independence or annexation.
March, 1846: General Taylor leads troops past the Nueces River toward the Rio Grande River, through and into the land that both the U.S. and Mexico claimed as its own.
April 25, 1846: The Mexican-American War begins when Mexican troops cross north of the Rio Grande River and opened fire on U.S. troops at Fort Texas.
May 8, 1846: Battle of Palo Alto - The first official battle of the war. After Mexican troops retreat, Taylor declares his army victorious.
May 9, 1846: Battle at Resaca de la Palma- The day after the Battle at Palo Alto, Taylor’s troops follow Mexico’s retreating men to Resaca de la Palma (resaca is the Spanish term for a dry riverbed) and were victorious again.
May 13, 1846: Congress officially declares war on Mexico.
May 18, 1946: General Taylor and troops occupy Matamoros. A few days prior, Mexican troops had left Matamoros quietly and retreated to Monterey.
Summer 1846: Thousands of American volunteers were authorized by Congress for twelve-month military service and arrived in the lower Rio Grande Valley shortly thereafter. “Most Americans enthusiastically supported the war. Approximately 75,000 men eagerly enlisted in volunteer regiments raised by the various states. Thousands more enlisted in the regular U.S. Army. There was no need for a draft. In some places, so many men flocked to recruiting stations that large numbers had to be turned away.
June, 1846: American settlers in California arrest Mexican politician Mariano Guadalupe Vallejo, part of what is referred to as the "Bear Flag" revolt. The Americans write a constitution and declare California an independent republic.
July 7, 1846: Battle of Monterey- General Taylor’s army followed the Mexican troops to Monterey and fought to take control of this city. “The fighting was fierce. In the end, it came down to hand-to-hand combat in the city streets. But again, the American army was victorious.
August 14, 1846: U.S. Colonel Stephen Watts Kearney, whose first mission was to occupy New Mexico, rode with troops into its capital, Sante Fé, and finding it deserted, easily took possession.
August-September 1846: Many U.S. volunteers assemble under General John E. Wool, who lead them to go join General Taylor and his troops.
December, 1846: Santa Anna is officially elected president of Mexico once again.
January, 1847 : Los Angeles is occupied by U.S. troops again and the Californians surrender.
Feb 22-3, 1847: Battle at Buena Vista- Generals Taylor and Wool and their troops were largely outnumbered when they met General Santa Anna and troops near Buena Vista. Yet, they still claimed the victory, one that earned General Taylor much attentioan and respect.
March 1847: General Winfield Scott’s “Army of Invasion” landed at Vera Cruz on way to seize the capitol- Mexico City.
April 19-20, 1847: Battle of Cerro Gordo- General Winfield Scott and his outnumbered troops fight through Santa Anna’s army and have an open path to Mexico City.
Summer 1847: Sent by President Polk to negotiate peace with Mexico, U.S. diplomat Nicholas Trist is unsuccessful at reaching an agreement with Santa Anna, and thus, the war continues.
Sept. 12-14, 1847: Battle of Mexico City- General Winfield Scott and troops arive in Mexico's capital city and fight their final battle against a wearied Mexican army. The victory and occupation of the city by the U.S. on Sept. 14 marks the unofficial end of the Mexican-American War.
January, 1848: A peace agreement that assigned the U.S. to pay Mexico $15 million dollars in receipt of California and New Mexico, the Rio Grande as the Texan border, and the right for Mexicans in those territories to become U.S. citizens was made.
Feb. 2, 1848: Mexican and U.S. negotiators (Trist) sign the Treaty of Guadalupe Hidalgo, it is ratified by the U.S. Senate on March 10th and by Mexican Congress on May 25th.

http://umich.edu/~ac213/student_projects06/magsylje/timeline.html