All of these posts are mentioning ancient generals, undoubtedly the best during their time, but how would they fare in the 16th/17th century fighting? Who knows? Therefore I believe the greatest general is Frederick the Great. He managed not only to defend the small nation of Prussia, with something like 1 million total soldiers under command, versus three huge nations(France, Russia, and Austria) with armies that easily dwarfed any he could bring(don't know exact figures right now but can get them) against them. Frederick managed to do this for 7 straight years. Although he was near defeat at the end and was saved by the ascension of Czar Paul II to the throne, which stopped the war with Russia, he was still, I believe, the best general before 1800.