Crossing the Rio Grande, the United States army continued its advance into Mexico. Thousands of American volunteers were incorporated into the U.S. military serving alongside the regular army, including Thomas Hamer, who had nominated Grant to West Point. Starting in September, Taylor and his Army of Invasion, moved south and engaged the Mexican army at the Battle of Monterrey. During the battle, Grant demonstrated his equestrian ability, carrying a dispatch through Monterrey's sniper-lined streets on horseback while mounted in one stirrup.