The war of 1847-8 was the first time since the War of 1812 that the US armed forces had to face an enemy universally equipped with firearms and trained in European style warfare. The Mexican army was, however, poorly regarded and most Americans expected an easy victory.
The war had its origins in the 1836 conflict that brought independence to Texas and was famous for the siege of the Alamo. The Mexican government had never accepted Texan claims to the land between the Rio Grande and Rio Nueces. In 1845 Texas became a state of the USA, so the USA inherited the border dispute.
President Polk moved 3,500 regular soldiers under Zachery Taylor up to the Rio Nueces. He also sent John Slidell to Mexico City with an offer to pay $25 million dollars for the disputed land in Texas, plus the Mexican provinces of Alta California and Santa Fe. The Mexicans turned down Slidell’s offer in brusque terms, so the envoy returned to Washington in disgust.
In Texas, Taylor had built a small fort in the disputed territory. On 25 April 1846 the Mexicans ambushed a patrol of US cavalry in the disputed lands, killing 11 men and capturing the rest. Polk told Congress that the Mexicans had “shed American blood on American soil” and on 13 May war was declared.
Meanwhile, General Mariano Arista led 3,400 Mexicans to attack Taylor’s fort, prompting him to march his main force over the Nueces to relieve the surrounded men. The resulting Battle of Palo Alto was dominated by artillery. Taylor used a battery of horse artillery – light guns pulled by fast horses – to hammer the Mexican army, which then fell back into broken country where the horse artillery could not operate. The Mexicans opened fire with their own heavier guns, halting the American advance. Next day Arista retreated over the Rio Grande.
Meanwhile, in Alto California, local US settlers had taken matters into their own hands, declaring the province to be independent of Mexico. A force of 60 US soldiers under Captain John Fremont was diverted from Oregon to California to support the American settlers against the local Mexicans. The Americans won the climactic Battle of La Mesa on 9 January, after which the Mexicans retreated from California.
from THE ATLAS OF THE WORLD AT WAR by Rupert Matthews