This happens a lot. I offer an opinion. I get lots of persuasive email telling me I’m wrong. I retract or modify my position. Then I get hit from all of the people who agree with me in the first place. I am officially out of the “decimate” business. I will await Derb’s definitive answer on this question. In the meantime I offer this reader’s reaction to my capitulation:

I’m with you on ‘Decimate’
If it weren’t for occasional fits of pedantry, we’d be lazily allowing our world-conquering language lose all its precision and fine shades of meaning.
Plus, I didn’t suffer through two years of college Latin to graciously suffer peoples’ blithe ignorance of the root meaning of words like this.
So fire away.
And regarding your anti-pedant correspondent:
“an army can be “annhilated” without being reduced to nothing”
Perhaps, but if the army didn’t at least come pretty darned close to being reduced to nothing, the word is being used incorrectly. Saying that a Republican Guard division that was reduced by 50% has been “annhilated” would be worse than calling it “decimated”. What in the heck is wrong with “halved”?