Napoleon considered him one of the seven greatest commanders in history. Clausewitz called him the father of modern warfare.
Gustavus Adolphus proved that a small nation with limited resources could defeat a superpower through superior organization and technology. He professionalized warfare, moving it away from the mercenary chaos of the Middle Ages into the disciplined statecraft of the modern era.
"He made his soldiers feel that they belonged to an order of monks, but an order of monks equipped with muskets."
His death at Lützen was a catastrophe for the Protestant cause, but his military reforms survived. The armies of Cromwell, Turenne, and eventually Napoleon were all built upon the foundation Gustavus laid.