He was a son of Emperor Suzong who was credited with suggesting to his father the idea of fleeing to Lingwu during the Anshi Rebellion, allowing Emperor Suzong to eventually reestablish a power base and defeat the rebellion.
However, he offended Emperor Suzong's favorite concubine Consort Zhang and the powerful eunuch Li Fuguo by accusing them of crimes; in turn, they accused him of having designs on the life of his older brother Li Chu the Prince of Guangping, causing Emperor Suzong to order him to commit suicide.
During Emperor Xuanzong's Tianbao era (742-756), Li Tan was created the Prince of Jianning, and was known for his decisiveness and abilities.
After the incident, Emperor Xuanzong was intent on continuing to Chengdu (the capital of Jiannan Circuit).
Now, if you follow the Supreme One [(i.e., Emperor Xuanzong)] into the Shu region [(i.e., Jiannan)], then the region east of the San Pass [(散關, in modern Baoji, Shaanxi)] will no longer be possessed by the imperial house, and how can you then maintain the support of the people?
Once Li Heng reached Lingwu and gathered the troops there, at the urging of the generals, he took imperial title (as Emperor Suzong).
On one occasion, when he praised Emperor Suzong for following Li Mi's recommendation to strip a jewel-studded saddle that Emperor Xuanzong gave to Consort Zhang as a gift of its jewels and give the jewels to the soldiers as rewards, Consort Zhang became angry and bore grudges against both him and Li Mi.
[2] He also posthumously married Lady Zhang, the daughter of his aunt Princess Xinxing,[3] to Li Tan, honoring her as Empress Gongshun (恭順皇后).