What allusions did Du Fu use in his Northern Expedition?

The Northern Expedition is a long narrative poem written by Du Fu, a poet in the Tang Dynasty. This poem was written in the second year of An Shi Rebellion, that is, in August of the second year of Zhide (757), on the way from Fengxiang to Zhangzhou, describing what he saw and felt after returning home. The then Prime Minister Gui Fang was accused of taking bribes by Helan Jin Ming and others. Du Fu, as a left gleaner, was angered by pleading guilty, and then the third division pleaded guilty and was rescued and released by Zhang Gao. In August, Su Zong ordered him to leave Fengxiang to visit relatives, which was actually sent back to China. The whole poem takes the personal experience on the way home and after home as the theme, describes the scenes of people's livelihood and national chaos during the An Shi Rebellion, and states his views on current affairs. The poet adopts the method of giving priority to fu and combining metaphor with xing, which shows the grand historical content and shows the poet's high talent and mastery in poetry art.