On this day, 645 years ago, on April 16, 1380 (March 11, the 1380 lunar calendar), Zhu Di became a vassal in Peiping. In 1368, Zhu Yuanzhang ascended the throne and became emperor, firmly controlling military and political power in his hands. From 1370 to 1391, he made 23 of his 26 sons princes three times and stationed them in strategic locations across the country. Zhu Yuanzhang stipulated that princes stationed in various places were not allowed to interfere with local civil affairs. As the emperor's representatives in local military power, they were the monitors of the local garrison forces and held the local military command power. In 1370, Zhu Yuanzhang's fourth son Zhu Di was granted the title of King Yan. In 1378, Zhu Yuanzhang changed Nanjing to the capital and let the princes be assigned to their respective fiefs one after another. On April 16, 1380 (March 11, the 13th year of Hongwu of the Ming Dynasty), King Zhu Di of Yan officially went to his fief in Peiping and took over. Because Peiping was shouldering the important task of defending against the harassment of the Mongolian aristocrats, Zhu Yuanzhang gave him more military units than other princes, and allowed him to make decisions on small matters in the army in addition to reporting major matters, which greatly increased Zhu Di's strength.