The great military theorist and philosopher Carl von Clausewitz had contempt for the plethora of manuals on the military art during the Enlightenment and proposed his own science of war that lacked the rigidity (and often the clarity) of these manuals. The dominant feature of many of these manuals was a search for a “system” that purported to provide the key to victory in war. This concept of system is often associated with the great fortifier Vauban who was declared to be the author of a number of “systems” of fortification. I will suggest that Vauban's reputation and the theories of fortification ascribed to him were influential in inspiring the belief that a rational and teachable art of war based on first principles was possible. The example of the technology of fortification and siegecraft introduced a new approach to writings on war in general the late eighteenth century. It is an example of the influence of things on ideas that were extrapolated beyond their originating locus. There is irony in this because in spite of Vauban's reputation he was not the man of systems that many of his emulators and disciples thought he was. In part this misconception arose from a reputation that dazzled and blinded and in part because of the success of the military engineering school at Mézières whose courses on fortification added to the prestige of the master. Clausewitz conceded that writings on siege warfare had provided the germ of the science of warfare as he envisioned it but that they also imposed a limit to an intellectual approach that he claimed to overcome in his own work.