The annexation of Texas took place in 1845 after the conquest over Mexico in the Mexican-American war.
Mexico was reduced to half its size and the USA increased by a third.
Some people debated over whether to expand slavery to the west.
This issue had supposedly been settled by the Missouri Compromise of 1820. David Wilmot, a Pennsylvania Congressmen, said, “Neither slavery nor involuntary servitude shall ever exist in any part of said territory”
Northern and southern democrats went against Northern and southern Whigs.
It was that conflict that eventually lead to a civil war.
Some historians state that the civil war was for nothing because the plantation owners wouldn't be able to expand their crops to the west because of climate conditions.