Final answer:
Juan Seguin fought on the side of Mexico in the war because he believed in the independence and sovereignty of Mexico and saw the American invasion as a threat.
Step-by-step explanation:
Juan Seguin fought on the side of Mexico in the war because he was a Tejano who sympathized with the Mexican cause. As a Mexican American, Seguin believed in the independence and sovereignty of Mexico. He believed that the Mexican government represented the interests of the Mexican people and saw the American invasion as a threat to Mexican sovereignty and land. Seguin fought alongside Mexican soldiers and civilians to defend their land from the American invaders, just like other Mexicans who fought bravely to protect their country.