Easy to win the state, hard to keep the system
Yang Jian ruled as regent in the name of being the empress dowager’s father, and received the abdication and took the throne in barely ten months, so that historians said "no state was ever won more easily than the Sui." But he knew well how he had come to power, and so labored to reinforce the institutions: appointments and dismissals of officials taken back into the Ministry of Personnel, local aides named by the center, annual evaluations. He guarded against every powerful minister, and guarded only against the wrong man — his own son.
Empress Dugu and the deposing of the heir
Dugu Qieluo and Yang Jian were called "the Two Sages," and agreed the emperor would father no child by another woman — a monogamous emperor, the only case in history. The heir Yang Yong kept many favorites and lived in luxury, which both of them despised; Prince Yang Guang carefully feigned frugality and devotion, dust on his zither and no concubines attended. In 600 Yang Yong was deposed and Yang Guang moved into the Eastern Palace. Four years later, gravely ill, Emperor Wen at last saw this son’s true face, too late — the fate of the Sui was already written in this choice of heir.