When were Du Fu's nine poems written in the frontier fortress?

Nine Frontier Fortresses is a series of military poems written by Du Fu at the age of forty in the 11th year of Tianbao (752). This period was also the growth period of the Tang Dynasty, accompanied by the military expansion of the Tang Dynasty. Most of the court's predictions were optimistic, but Du Fu didn't quite agree with the military line of Emperor Xuanzong of Tang Dynasty.

Zhu said that these nine poems were written for Geshuhan's fighting in Tubo in the last years of Tianbao. After Emperor Xuanzong of the Tang Dynasty ascended the throne, in order to satisfy his desire for great success and plunder wealth, he constantly launched unjust wars in border areas. In the sixth year of Tianbao (747), Dong attacked Tubo Shibao City; In the eighth year of Tianbao (749), Ge led 100,000 troops to attack Shibao City again, and most of the soldiers died in battle, with rivers of blood. In the tenth year of Tianbao (75 1 year), Jiannan was ordered by the Central China to attack Nanzhao, and 60,000 people died. Gao Xianzhi attacked the big food and An Lushan attacked the Khitan, and the people in both places suffered greatly. This group of poems was created under such a historical background.