It would definitely be China. Territorial China as we know it today was not a single sovereign country arguably until the 19th century, and some would even say the 20th Century (beginning 1911 under Sun Yat-sen). China defines its history according to the history of warfare and the strategy of warfare. The most famous (although not really the first if you look deeper and find Sun Pin) treatise on war known as The Art of War is a seminal piece and is testament to the fact that China has more experience with armed conflict than any other country