The Mexican-American War (1846 - 1848) was provoked when President James Polk sent troops in to the bit of land between the Nueces River and the Rio Grande. The more northern Nueces had been the border of Texas before the Texas Revolution (itself sponsored by the United States) and was the border still recognized by Mexico. The Rio Grande was the border claimed by the Republic of Texas after it seceeded from Mexico, and as a result was the border recognized by the United States when Texas was annexed. By sending US troops in to the disputed area and having them start construction of a US fort (Fort Brown, on the north bank of the Rio Grande), the US forced Mexico in to a war it was ill prepared to fight.
The opening hostilities, according to US history, was the "Thornton Affair," named for one of the American commanders, Captain Seth Thornton. Thornton led a company of Dragoons on a scouting mission. While investigating a seemingly abandoned hacienda on April 25, 1846, the company discovered an encampment of some 2,000 Mexican soldiers under the command of Colonel Anastasio Torrejon. In the ensuing battle, 16 of the American Dragoons were killed before the greatly outnumbered company surrendered. The surviving Dragoons were taken to Matamoros, Tamaulipas, and held as prisoners of war. Upon hearing of the capture, President Polk declared that "American blood had been shed on American soil" and ordered Congress to issue a declaration of war, which they did gladly.
Mexico, citing the established border of the former province of Texas, asserted that the United States had instigated the hostilities, first by encouraging the Texas Revolution and supplying the rebels, then by invading Mexican territory with a large force of troops. (
Map of the United States, 1840, with an overlay of current states.)
In the treaty that officially ended the war (the Treaty of Gadelupe Hildago) and several more that settled on-going disputes and simmering hostilities, Mexico lost about half of its former territory to the United States, including what is now New Mexico, Arizona, California, Nevada and Utah along with parts of Colorado, Wyoming, Oklahoma and, of course, Texas. All in all, the loss of life -- estimates range between 17,000 and 38,000 American and Mexican soldiers -- proved to be quite profitable to the real estate speculators, the railroad barons and the politicians who followed their orders, all of whom became the wealthiest men in late 19th century America.
Gee, no similarity to modern events there.