When the Civil War broke out in 1861, there were tens of thousands of Mexican Americans living in California, Texas and the New Mexico territory; all former parts of Mexico that the U.S. had claimed in the 1840s. With the wounds of the Mexican-American War fresh, these Mexican Americans now found themselves in the middle of the United States’ war with itself.

At first tejanos, aka Mexican Americans in Texas, “tried to avoid declaring support for either side,” writes Sonia Hernandez, a professor of history and Latino/a and Mexican American studies at Texas A&M University, in an email.

“Some outright avoided joining either side because tejanos were accused of disloyalty even before the war officially broke out,” she writes. “Tejanos could avoid conscription by claiming Mexican citizenship and some were in fact Mexican citizens. Still others, overwhelmed with the growing divide, chose sides.”

Jerry D. Thompson, a history professor at Texas A&M International University, estimates that a few thousand Mexican Americans joined the Confederate troops and over 10,000 joined the Union Army and Militia. Though there was some overlap, most Mexican Americans who joined the Union lived in the U.S. territory of New Mexico or the state of California, while most who joined the Confederacy lived in Texas, one of the states that seceded. At least 2,500 tejanos joined the Confederate Army.

Mexico had banned slavery in 1829, several years after it won its independence from Spain, and some Mexican-Americans may have joined the Union because they opposed U.S. slavery. “There is some evidence that there was a mini underground railroad here in south Texas that was largely fueled by tejanos, usually poor tejanos, who would help runaway slaves escape into Mexico,” says Thompson. “We know there were thousands of runaway slaves in Mexico.”

At the same time, there were wealthy Mexican Americans who owned slaves and those whose income depended on the slave trade. “You also had well-to-do individuals like Colonel Santos Benavides here in Laredo who actually became the highest ranking tejano officer in the Confederate Army,” says Thompson. “There are instances of him acting as a slave catcher, where he’s actually going into Mexico and retrieving these runaway slaves and returning them to their masters, for which he was compensated.”

The desertion rate among Mexican American Civil War soldiers was high, mostly because of the prejudice they experienced from white soldiers on both sides, according to the National Park Service. There was even one tejano captain, Adrián J. Vidal, who joined the Confederacy, deserted for the Union, then deserted again to fight against the French imperialists in Mexico who supported the Confederacy.

There were other reasons Mexican Americans may have wanted to join the Union. In the early 1840s, the white-run Republic of Texas had invaded New Mexico, then still part of Mexico, in an attempt to seize more land, so there was “a deep resentment in New Mexico of Texans,” Thompson says. The vast majority of Mexican Americans in the New Mexico territory who entered the war fought for the Union, which promised a bounty of up to $300 for soldiers.

In contrast, “the Civil War deeply divided the Mexican Americans of Texas,” Thompson writes for the Texas State Historical Association. Tejanos who joined the state’s Confederate militia units “frequently did so out of a fear of being sent out of the state and away from their families. Some were able to avoid conscription by claiming to be residents of Mexico.”

In Texas, tejanos who resented white Texans for taking away their land may have joined the Union in retaliation, Hernandez suggests. “Others simply wanted to remain in the area and it was easier if they supported the Union, to stay and protect their communities as opposed to being sent to other parts of the South,” she writes.

Mexican Americans who joined the Confederacy fought as far away as Virginia and Pennsylvania. But Mexican American soldiers in the Union fought closer to home, and helped secure key victories in the southwest.

In March 1862, Lieutenant Colonel Manuel Chávez led Union troops in an attack on a Confederate supply train at the Battle of Glorieta Pass. The Union victory helped push the Confederacy out of New Mexico for good, later earning Glorieta a nickname: the “Gettysburg of the West.” Today, the battlefield is part of Pecos National Historical Park, and a marker of Mexican Americans’ contributions to the U.S. military.