Which allusion does the idiom "morning and evening" come from?

Serve Qin in the morning and Chu in the evening-change sides quickly.

[Interpretation] During the Warring States Period, the two major vassal states of Qin Chu were opposed to each other and often fought wars. Some vassal States sometimes favored Qin and sometimes Chu for their own interests and security. Metaphor is capricious.

Allusions:

During the Spring and Autumn Period and the Warring States Period, Qin Chu, the two great vassal states, were opposed to each other and often fought wars. For their own interests and security, some small vassal States sometimes tend to Qin and sometimes to Chu.

Guanya is a veritable "land of mourning for Qin". It is conceivable that Guan Ya, who is in the crevice or buffer zone of Qin Chu, is often cruel and fierce because he is in the forefront of the war. In a pool of blood, the Qin people occupied the Chu land in Guanya in the morning, but in the evening, the Chu people regained the lost land. So many times, the former Pakistanis living in Guanya have to face it in order to survive.