Which was the last major land acquisition the united states mad prior to the beginning of the civil war?

what the first or next guy says

the federal constitution of the united mexican states of 1824 was enacted on october 4 of 1824, after the overthrow of the mexican empire of agustin de iturbide. in the new constitution, the republic took the name of of united mexican states. it was replaced by the federal constitution of the united mexican states in 1857.