It's really hard to throw out every other general and say "THIS IS THE ONE TRUE FATHER" because Sun Tzu, Clausewitz, Napoleon, Gustavus Adolphus all made significant additions to military thought. You can make a case for many generals although i would say the strongest case is for either Gustavus Adolphus or Clausewitz; however Clausewitz did more in terms of military philosophy were as Gustavus Adolphus did more to further actual military tactics, most notably with his application of combined arms. However both these generals are so far removed from "Modern War" that it gets hard to measure how much of modern military thinking can be attributed to a single "father".

The problem is how do you define "Modern Warfare"? The First World War by its end had combined arms, mechanized warfare, strategic bombing, war in the air and rapid action but many of these were in its infancy. The Second World War featured all of these but to a more developed extent. The Civil War brought in the importance of transport, production and communication as fundamental aspects of fighting.

If you want to understand History it helps to ignore things like "What was the first modern war?" or "who was the father of x?". Wars are not as clear cut. WW2 was similar to WW1 which had similarities with Franco-Prussia or the US Civil War. But its hard to group all of those together as there was so much different between the extremes.

Martin Gilbert "First World War" Heinz Gudarian "Panzer Leader" Rommel "Infantry Attacks"