Indeed. However, it is not called 'secession' but "Mexican Cession".
The Southwest, including California, was taken "by force of arms" from Mexico, during the Mexican War.
The Mexican American War.
A lot of this land was won by the United States in the Mexican-American War.
Keep slavery out of newly acquired Mexican territory
California and the American Southwest.
Southwest
the Mexican War
It gained California and the American Southwest.
They paid Mexico for the Southwest after defeating them in the Mexican War.
(1846-1848) The war between the United States and Mexico in which the United States acquired one half of the Mexican territory.
It was separated from the territory acquired at the end of the Mexican-American War and became a state in 1912.