Robbers, Rangers and Tigers
In 1859 Juan Cortina, nicknamed the "Red Robber of the Rio Grande," raided and occupied Brownsville, Texas. His reason? Reprisals for supposed persecutions of Mexicans by Americans.
When Texas seceded (the first time) from Mexico, land ownership became a controversial issue. Did Mexicans who happened to wind up inside the (hotly contested) lines of the newly formed republic still own their land? Or did it now belong to Texas citizens?
When Texas joined the United States as the 28th state, the problem didn't clear up. Poor Mexicans and Texans claimed they were being legally harassed out of their land by rich Texans. Border raids and violence between the factions raged. If you think there are bad race relations between Mexicans and Americans now, just imagine what it was like when there was a disputed border and little Federal Government out in the "Wild West!"
So what happened to Brownsville? Though Cortina was talked into leaving the town, Brownsville residents organized the "Brownsville Tigers" to counter Cortina's 40-80 man army and stop his border raids. But they were no match for Cortina. It wasn't until the Texas Rangers got involved (and on their second attempt, at that!) that he was temporarily defeated and sent back to Mexico.
"Texas Troubles"
Just about the time things settled down on the border, the summer of 1860 brought the "Texas Troubles." Fires suddenly broke out and destroyed large portions of Dallas, Denton and Pilot Point. At the time, there were widespread fears that these fires were part of an abolitionist plot to bring destruction and insurrection to Texas. Lincoln's election only confirmed their fears that abolitionists were shortly going to wreck the state.
And so we come to one of the main reasons (besides slavery and states rights) for Texas' secession: They felt they could handle protecting their state much better than the United States Federal Government.