It didn’t mean remove 1 in 10 it meant kill 1 in 10 soldiers as punishment. From (the best dictionary!) American Heritage 5th ed.:
Usage Note: Decimate originally referred to the killing of every tenth person, a punishment used in the Roman army for mutinous legions. Today this meaning is commonly extended to include the killing of any large proportion of a population. In our 2005 survey, 81 percent of the Usage Panel accepts this extension in the sentence The Jewish population of Germany was decimated by the war, even though it is common knowledge that the number of Jews killed was much greater than a tenth of the original population. This is an increase from the 66 percent who accepted this sentence in our 1988 survey. However, the Panel is less accepting of usages that extend the meaning to include large-scale destruction other than killing, as in The supply of fresh produce was decimated by the nuclear accident at Chernobyl. Some 36 percent accepted this sentence in 2005, up from 26 percent in 1988, but still a decided minority.