Guibert was France’s foremost military theorist in the eighteenth century. He wrote two great treatises, but the second is often ignored in favor of the first, the General Essay on Tactics. That second treatise, the Defense of the System of Modern War, is presented here for the first time in English, with extensive annotations by the translator. In it, Guibert explores the beginnings of operational-level warfare, laying the groundwork for Napoleon and his successors.