What key event led to American forces defending the Alamo building against the Mexican army?

The key event that led to American forces defending the Alamo was the Texas Revolution, which began in 1835 as Texas settlers sought independence from Mexican rule. Tensions escalated between the Texian (Texas) colonists and the Mexican government, culminating in the siege of the Alamo by Mexican forces under General Santa Anna in February 1836. The defenders, including notable figures such as James Bowie and Davy Crockett, aimed to resist the Mexican army's attempts to reclaim the fort, leading to the famous 13-day siege.