The oldest son of the Hongwu Emperor died before him, so the next emperor was his grandson the Jianwen Emperor. The Jianwen Emperor was about 20 years old and didn't like the power of his many uncles. He started taking away their power, sending them away, or even making them kill themselves. The uncle leading Beiping (old Khanbaliq and now Beijing) was named Zhu Di. He pretended to be crazy so that the Jianwen Emperor would be less afraid. The emperor even let his three sons—kept in the capital city to be sure of their father's good behavior—go north see him. Zhu Di then began a war against his nephew.