That was the original definition, modern usage is to reduce to 1/10th.
I'm fairly certain even that may not be enough.
Actually, the original definition (1528) was to levy the tithe (10% religious tax). According to the Oxford Dictionaries:
Decimation appears to be a slightly older word in English than the verb as it began to make an appearance in English writing in the early 16th century, some seventy years prior to decimate. Again, recent research can provide an earlier example than the current unrevised OED entry. It appears in a book by William Barlow, printed in 1528, where he writes ‘To forge excommunicacions For tythes and decimacions Is their continuall exercyse.’
If we look to the dictionaries of this time period the evidence suggests that this tithing sense of decimate was just as common, if not more so, as the sense of killing or punishing one of every ten. The first English dictionary to record the word was Thomas Blount’s magnificently titled Glossographia, published in 1656, which defines decimate as “to take the tenth, to gather the Tyth”, with no mention made of killing anyone, soldiers or otherwise. In Elisha Coles’ An English Dictionary, published some twenty years later, it is defined as both ‘to tythe or take the tent’ and ‘also punishing every tenth man’. These are the only two dictionaries of the 17th century to define decimate (which is not terribly surprising, as there were very few such reference works at the time).