Emperor Yang of Sui
The transition started roughly around the year of 613 when Emperor Yang of Sui launched his first of three campaigns against Goguryeo, leading to a number of desertions in the army and the start of agrarian revolt against the Sui. The transition ended in 628, when Emperor Gaozu's son Li Shimin annexed the agrarian rebel ruler Liang Shidu's state of Liang, thereby once again unifying most of China under a single power.