Such were the arts of war, by which the Roman emperors defended their extensive conquests, and preserved a military spirit, at a time when every other virtue was oppressed by luxury and despotism.

• 这种不涉及技术层面的字句就不要去推敲它了，不影响表达嘛，这是我的建议。且本来这个句子按汉语语法习惯就难以很好的翻成汉文。