@Reconstructo,
I would have to say
Carl Marie Von Clausewitz. Born in 1780, Clausewitz was a Prussian tactician who, although distinguished in service, his career was not entirely outstanding. However, Clausewitz offered perhaps one of the most valuable (and terrible) contributions to military strategy (and arguably philosophy) entitled
On War. There is still a great deal of contention between historians and philosophers as to the exact status of Clausewitz. Notable historians such as the Dupuy brothers have proclaimed Clausewitz "?the greatest philosopher of war" in
Encyclopedia of Military History. Especially considering the fact that Clausewitz is being compared to the likes of Machiavelli and Sun Tzu, there is clearly something special about him. Is he often thought of as a philosopher? I would say no, but from the perspective of military history, he has lent a great deal to the philosophy of war.
On War is essentially a cold, objective, and overall sterile look at war devoid of humanity and supporting a notion of absolute war. Clausewitz's views on war are in stark contrast to the forgiving nature of Sun Tzu and the ethically fluid nature proposed by Machiavelli. And Clausewitz is especially not your modern day Nicholas Fotion... far from it. Interestingly enough (from a philosophical perspective we may be acquainted with), Clausewitz was arguably influenced by Kant. For example, war should, for example, be put in its proper perspective and the notion of absolute war was a thing in itself and studied as a model to emulate.
Unfortunately, many historians attribute the logical outlook Clausewitz held on war as influences in Germany's aggressive military progression. Bismark was undoubtedly influenced by Clausewitz (through Moltke). Hitler was captivated by the military modus of which Clausewitz proposed (for Clausewitz day), containing huge sections of Bonapartism as well as a theory of absolute war that lent to the formation of Hitler's conception of Blitzkrieg. There were also archaic notions of war proposed by none other than Genghis Khan that influenced Clausewitz. Khan is well known (hilariously through the Conan the Barbarian movies) for his view on war, which was that, "? the greatest joy is to vanquish enemies, chase them, tear them, tear the wealth from them, watch them who love them weep and sexually enjoy their wives and daughters."
Here are a few neat quotes from Clausewitz to get more of an idea of what his philosophy on war was like;
"To introduce into the philosophy of war a principle of moderation would be absurd."
"Let us not hear about generals who conquer without bloodshed."