Some American soldiers fighting during World War II discovered that months of separation from their hometown sweethearts could lead to unfortunate personal events. One such event involved receiving a formal and terse break-up message from home called a Dear John letter. A Dear John letter often began with a formal or perfunctory greeting, not the usual "My Dearest Sam" or "My Sweet Darling," which served to let the recipient brace himself for bad news. The contents of a typical Dear John letter (if there were any contents) would also be direct and detached: "I have met someone else since you've been away and I believe it would be best if you and I agreed to part company."

The origin of the "Dear John" in a Dear John letter is still a matter of controversy. Some sources believe the name John was chosen because of its commonality at the time, much like John Q. Public or John Doe is used today. Others say that "Dear John" was a reference to several popular songs which referred to foot soldiers as "Johnnies," as in When Johnnie Comes Marching Home Again. There is also a theory that a popular 1930s radio show began each episode with a female actress intoning dear John as she began reading a letter to her unknown paramour.

The practice of sending a Dear John letter as a long distance break-up tactic became so common, in fact, that some women didn't even bother to compose more than the salutation. A soldier might only receive the message "Dear John" and nothing else. When fellow soldiers pressed the recipients for more details, many replied "That's all she wrote." This is said to be how the phrase that was all she wrote came into popular usage.

The Dear John letter may have been replaced by the Dear John e-mail or the Dear John phone call, but it is still a sad reality for some members of the military serving away from home. There is also the Dear Jane letter, which affects female military members in the same way as the Dear John letter. Some in the business world also refer to formal letters of dismissal as Dear John letters, since they serve essentially the same function for a company's soon-to-be former employees.