and placed Taiyuan under siege, but was ultimately forced to retreat after he struck against the defences of the Northern Han with the Liao cavalry coming in to support.Emperor Taizu established the core Song Ancestor Rules and Policy for the later Song emperors. He was remembered for his expansion of the imperial examination system such that most of the civil service were recruited through the exams (in contrast to the Tang where less than 10% of the civil servants came through exams). He also created academies that allowed a great deal of freedom of discussion and thought, which facilitated the growth of scientific advance, economic reforms as well as achievements in arts and literature. Emperor Taizu is well known for bringing the power of the military under control, ending the era of the warlords, centralizing the state over regional commanders and so preventing anyone else rising to power as he did. Upon becoming emperor, he invited the general officers to a lavish banquet, where he convinced them all to retire as military leaders or accept minor posts, in favour of enjoying extensive estates and generous retirement funds and benefits which he then offered them. At a certain point during the feast, the new emperor made a speech to the military officers assembled there, which he began by expressing his deep gratitude to each and all of them for placing him on the throne, and that now that he had the power to do so, he wished to reward them to the utmost of his ability; then he went on to say that he thought the present company would all understand that he could not feel at ease on his new throne, with them continuing in command of their various armies of troops: and, he said, that if they duly considered the ramifications of the matter, neither would they. He then sincerely promised that they and their families would live in happiness and harmony, if they accepted his offer to retire with the stated benefits: eventually, none of the generals refused his terms, and thus began a period of relative internal peace within the realm for the duration of the Song dynasty which he thus founded, also better securing the military forces for involvement with the rival surrounding empires. Many Song and later sources record the story of the "Taizu's Oath", which forbade his successors from killing scholar-officials. However, this story