On this date in 1402, the second Ming emperor Zhu Yunwen (朱允炆 b. 1377) died in a palace fire. Reigning as the Jianwen Emperor (建文 r. 1399-1402), Yunwen as the grandson of Ming founder Zhu Yuanzhang (朱元璋 r. 1368-1398). After taking the throne, Zhu Yuanzhang enacted a series of reforms/edicts, one of which was to mandate the imperial line of succession be the first son of the emperor. (One suspects Yuanzhang just wanted to avoid a terrible headache–the man had 24 sons after all.)
Zhu Yuanzhang’s first son, Zhu Biao 朱標, predeceased his father, and so according to the code, Zhu Biao’s first son became the new crown prince, by-passing his 23 uncles. Not good.
One of those uncles, Zhu Di 朱棣, was a talented commander and a cunning strategist. Perhaps a bit too cunning. Zhu Di had been dispatched north to the area around the 
old Yuan Dynasty capital of Dadu (present day Beijing) to keep an eye on the Mongols, who were still spoiling for a fight.
Zhu Di was already unhappy and Yunwen, upon taking the throne, poured gasoline on the fire by taking a series of steps to limit the powers of the regional commanders, especially his uncles, and barred Zhu Di from visiting the tomb of Zhu Yuanzhang.
Fed up with his nephew, Zhu Di sent his armies south to Nanjing, capturing that city in 1402 and sought to depose Zhu Yunwen. Before he got the chance, Yunwen and a bevy of his consorts supposedly died in a mysterious palace fire. (Though rumors of his nephew’s miraculous escape and exile would plague Zhu Di throughout the latter’s reign as emperor)
Following his conquest of Nanjing, Zhu Di traveled to the tomb of his father, and on July 17, 1402 was crowned emperor, marking the start of the Yongle Reign (永樂 1402-1424).
It might have been for the best. Yongle was an energetic and capable emperor whose influence in Chinese history among the Ming emperors ranks right up there with his father, the dynastic founder. Following his usurpation of the throne, Zhu Di feared that Nanjing harbored too much ill will and could be a breeding ground for gossip and conspiracies. Leaving a token bureaucratic establishment in the original Ming capital, Zhu Di moved his court north to Beijing, which remained the center of power–however nominally at times–until 1927.
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Image: Zhu Yunwen, The Jianwen Emperor