Perhaps we could have something similar to the Stono River Rebellion that occurred in the early 1700s? In that Rebellion, slaves took control of a general store, stole guns and ammunition, and attempted to flee to Spanish Florida, freeing fellow slaves along the way. When confronted by South Carolina militia, they actually put up a very good fight before being defeated.
Perhaps slaves in Texas steal weapons, revolt, and flee to Mexico, gain support from the Mexican Government and try to free more slaves?