What two classics do the first four sentences of Cai Wei's poems imply?

In the past, I left, and Liu Yang Yiyi. Today, I think it is raining. It was still spring when I came out, and it was already snowy winter when I came back. Judging from the content of Cai Wei, it was an era when soldiers defended their country and returned to work. This poem praises the hard life and homesickness of the soldiers who joined the army.