The US president declared war on Mexico after the kidnapping of the American soldiers in the American land. This was after the US president had sent a Spanish ambassador to negotiate over the purchase of Texas, California, and New Mexico. Mexico had refused to relinquish the three regions, but the US took them forcefully through the Mexican-American War. The major problem was how the newly acquired regions could be shared out between the south and the north.
The north feared that these states could end up becoming slave states, which would increase the number of southerners in the senate. This hostility led to tensions that later led to the Civil war. In this regard, the Mexican war is closely related to the Civil War because it actually fuelled the civil inconsistencies. British troops defeated them and signed a Proclamation Treaty in 1763, which barred colonists from settling in the western lands.
The British did not want any conflicts with the Indians at the western lands because the Seven Year War had cost a lot. However, the British colonists refused to obey a government order and invaded these places, which led to conflicts between the Indians and the British government. With time, this led to the American Revolution that granted independence to the locals.