During the Han Dynasty, professional soldiers existed in the sense that we know them: hired men who worked for the government. A personal favorite article is Edward Dreyer's posthumously published "Zhao Chongguo: A Professional Soldier of China’s Former Han Dynasty." (PM me if you'd like a copy) It's a narrative taken from the soldier's memoirs that's a great look at the military of the time. This was not always the case- after the fall of the Han emperors, a number of folks vied for control of various parts of China. The Tuoba tribe of the Mongolian Xianbei, for instance, were able to conquer much of northern China with their formidable cavalry. During this period, the military essentially was the government; its power started to wane as it began to rely more on local soldiers in its ranks. It struggled to adapt to urban administrative tasks foreign to the semi-nomadic steppes-folk. (Six Dynasties Civilization, Albert Dein) I can also reccomend Peter Lorge's edited volume "Warfare in China to 1600" as a good selection of articles on the nature of the Chinese military.