In the Shadow of the Alamo — Sherry Garland

Fifteen-year-old Lorenzo Bonifacio never intended to be a soldier. But when the soldados raid his tiny Mexican village, Lorenzo finds himself forced to join General Santa Anna's army . . . all because of the pesky goatherd Catalina and the haughty Esteban Equivel, son of the wealthiest landowner in the region.
Taken under wing by the kind Sergeant Ildefonso, Lorenzo and Esteban endure boredom, exhaustion, and hunger as the army makes its way across Mexico to San Antonio de Bexar, where rebellious norte-americanos have barricaded themselves inside an old mission called El Alamo. Neither boy can imagine what lies at the end of the march--defeat, death, glory?--and neither can accept the price paid during the fight for Texas.