Decimating an army has commonly meant killing off so many soldiers that the army is no longer an effective fighting force. However, there are actual numbers assigned to decimation.

Believe it or not, an army's own commanders are the ones who decide to decimate their army. In these terms it means to kill or execute one out of every ten soldiers. This usually happens after the non-commissioned officers are executed for incompetence.

During World War I the French army mutinied and refused to go "over the top" in attack; they did defend against German attacks. Although this happened at a particularly bloody battle, the mutiny was a long time coming, the result of years of bloody and wasteful tactics against a well-entrenched German army. The French troops were tired of being thrown at wire and machine gun nests in massive human wave attacks which cost thousands of lives. Small unit tactics had been successfully tried but the French commanders favored a more traditional (i.e. wasteful) tactic.

The French command executed all Non-Comms (Non-commissioned officers) and then shot every tenth soldier in an attempt to force the troops back into action. The tactic did not work; the soldiers realized that they had a better chance at one in ten with the decimation than they did against the German trenches where only one in ten would survive.

The political leadership of France relieved the high commanders of the army and replaced them. The troops went back to the front with new leadership and continued to fight.

Source: "The World Wars" as taught by Bob Welsh at Westminster College

Reply: If any information in my writeup is not factual, I apologize. It as been some time since I took the class and am recalling the information as it was presented to me (yes, I did pull out my class notes). As I remember, the records of the mutiny were sketchy, and I have heard various versions of the same events. The writeup below is clearly more detailed and cites various sources. My intention was simply to set the record strait about this commonly misused word. The example of the French Army was merely an illustration of the concept.
Thank you, Kung for clearing up this node, and setting me right again.