"They" is not acceptable as an impersonal pronoun? Since when?
No, because "they" means "more than one". This one actually drives me nuts, and I have no idea why. I noticed it immediately in the original post but figured I'd let Bitt do his thing.

Usually you can rewrite a sentence so that "they" becomes acceptable and you don't need "he or she", but unfortunately in this case I don't see a way around it.
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-Jeff
Rome did not create a great empire by having meetings; they did it by killing all those who opposed them.