Here's my answer on the unification of the Song dynasty.

In terms of bringing the military under control, Peter Lorge, who I cited in my original answer, also has a piece titled The Entrance and Exit of the Song Founders detailing how Zhao Kuangyin famously convinced his generals to retire and thus solidifed his position as emperor (though as I mentioned, at the time of this declaration, he was merely ruler of the North; not of a unified China).

Essentially, Lorge makes the argument that prior to the retirement, the title of "emperor" was more like a military rank rather than the title of the ruler; thus, the removal of the emperor and the advancement of a new emperor (presumably from one of the generals who held great military power) would help advance any subordinate looking to gain more power. This lack of legitimacy and power arrangement meant that the seat of the emperor was always under threat (from the generals that he trusted regardless of their own opinions on the matter) by the mere presence of these generals. By retiring and resting on their laurels, these generals solidified their own positions and wealth and also helped Zhao Kuangyin build a legitimate regime and create a distance between the Emperor and his subjects. Marriage into the Imperial family helped assure these generals that it was a good decision, and the experiences that Zhao Kuangyin and his comrades had been through together helped ensure that his generals trusted him on the matter.