The trigger-happy state police opened fire without warning on a crowd of Saturday shoppers in Brownwood on Aug. 19, 1871.

In his inaugural address in April 1870, Republican Edmund J. Davis asked fellow Radicals for a new law-enforcement legion to bring Reconstruction order to the chaotic countryside. Two months later, the governor got his wish – hired guns who answered only to him.

The state police was made up of quasi-autonomous companies that often acted as judge, jury and executioner. The...