How did the Mexican-American War start?

The Mexican-American War started due to territorial disputes between the United States and Mexico after Texas's annexation.

The Mexican-American War, which took place from 1846 to 1848, was primarily a conflict over territory. The immediate cause of the war was the United States' annexation of Texas in 1845. Texas, which had won its independence from Mexico in 1836, had initially sought to join the United States, but its admission was blocked by northern states in the US Congress who were against the expansion of slave states. However, in 1844, James K. Polk, a strong advocate of westward expansion, was elected President of the United States, and he pushed for the annexation of Texas.

Mexico, which had never officially recognised Texas's independence, considered the annexation an act of aggression. The Mexican government severed diplomatic relations with the United States, but the Polk administration was undeterred. Polk believed in the concept of 'Manifest Destiny', the idea that the United States was destined to spread across the North American continent, from the Atlantic to the Pacific. He sent troops to the disputed area between the Nueces River (the border claimed by Mexico) and the Rio Grande (the border claimed by Texas).

In April 1846, a skirmish broke out between Mexican and American troops, with several American soldiers killed. Polk used this incident to argue that Mexico had "shed American blood on American soil", and he called for war. Despite opposition from some quarters, particularly from those who saw it as an attempt to expand slave territory, the US Congress declared war on Mexico in May 1846.

The Mexican-American War was a significant event in the history of both countries. For the United States, it resulted in the acquisition of vast territories in the west, including California and New Mexico. For Mexico, it was a devastating defeat that resulted in the loss of about one-third of its territory. The war also exacerbated sectional tensions within the United States, setting the stage for the American Civil War.