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Tian Ji'an was born in 781 or 782, during the reign of Emperor Dezong of Tang. At that time, his father Tian Xu was an officer under Tian Xu's cousin Tian Yue, the military governor of Weibo Circuit, who governed the circuit semi-independently from the imperial regime. (Tian Xu's father Tian Chengsi had been the first military governor of Weibo, but chose Tian Yue to be his successor rather than any of his sons.) Tian Ji'an was Tian Xu's third son, and was born of a mother of lowly birth. After Tian Xu assassinated Tian Yue and succeeded him in 784, Emperor Dezong gave Tian Xu his daughter Princess Jiacheng in marriage in 785. As she had no son of her own, she adopted Tian Ji'an as her son. He was thus made the deputy military governor and Tian Xu's designated successor. When Tian Xu died suddenly in 796, the Weibo soldiers supported Tian Ji'an as acting military governor, and Emperor Dezong made him military governor later in the year. Tian Ji'an also inherited Tian Xu's title as Prince of Yanmen.