in 1845, texas, which was a nation after it left mexico, joined the united states. mexico did not like this, and the americans wanted the land mexico had on the west coast (“manifest destiny”). this led to the u.s. and mexico fighting a war called the mexican-american war. during the war, the u.s captured the cities of san francisco, los angeles, monterrey, veracruz and mexico city. as a result of the war, the u.s. gained land in california and much of the american southwest. many people in the north did not like this war, because they thought it was just good for southern slave states.