Jack reeeived a dear John letter from his girl friend because he had broken her heart

杰克收到了一封他女朋友给他的分手信,因为他伤了她的心.
这里有一个借代,a dear John letter （给约翰的一封信）指的是分手信
a dear John letter 是美国的一首歌,内容就是一个女孩要跟她的男朋友分手.
附a dear John letter 歌词：
Dear John
oh,how I hate to write
Dear John
I must let you know tonight that my love for you has died away like grass upon the lawn.
And tonight I wed another,dear John.
I was overseas in battle when the postman came to me.He handed me a letter and I was just as happy as I could be,because the fighting was all over and the battles had all been won.But then I opened up that letter and it started:Dear John.
Won't you please send back my picture?My husband wants it now.When I tell you who I*m wedding,you won*t care,do you anyhow?And it hurts me so to tell you that my love for you is gone.But tonight I wed your brother,dear John.