The US troops who fought in the Mexican-American War were primarily

Mexican immigrants.

enslaved workers.

enlisted men.

volunteer soldiers.

The US troops who fought in the Mexican-American War were primarily volunteer soldiers.

Volunteer soldiers.

Explanation:

The Mexican-American War was a result of the incorporation of Texas territory to the United States in 1845. This war was fought by 35,000 troops from the U. S. Army and 73,000 volunteers from different states. The majority of regiments which volunteed belonged to states from the south, such as Louisiana, Tennessee, Missouri, and Texas. The conflict began in April 25, 1846 and finished in February 2, 1848.