Sentence:  ...where Caesar threatened to disband "Legio X" after a mutiny. The men begged him to <WSD>decimate</WSD> them instead, and Caesar relented in the same way that Titus refrained from executing this cavalryman after his comrades' appeal. 
Meaning of the word marked with <WSD> tags:  To kill one-tenth of a group,  as a military punishment in the Roman army selected by lot, usually carried out by the surviving soldiers.