[PsyOp]

I think you're right. Much of it is based on conversations he had with a prominent Italian military figure of his time (I forget which one) as well as some other associates of his. I don't think he had any first hand military experience.

[Cacique]

The following from Clausewitz.com is a partial biography. He did have extensive combat experience during the Napoleonic wars.

Carl Phillip Gottleib von Clausewitz (1780-1831) was a Prussian soldier and intellectual. He came from a humble social background, though his family claimed nobility. He served as a practical field soldier (with extensive combat experience against the armies of the French Revolution and Napoleon), as a staff officer with political/military responsibilities at the very center of the Prussian state, and as a prominent military educator. Clausewitz first entered combat as a cadet at the age of 13, rose to the rank of Major-General at 38, married into the high nobility, moved in rarefied intellectual cirles in Berlin, and wrote a book which has become the most influential work of military philosophy in the Western world.

[Sawdring]

Vegitius was a Roman who gathered ancient writings to help Rome defeat her enemies. Offhand I can't remember what century it was written in but I think it was during Rome's decline.