From some old sheet music I came across. While the exact origins of the phrase are unknown, it is commonly believed to have been coined by Americans during World War II.  As letters to servicemen from wives or girlfriends back home would typically contain affectionate language (such as “Dear Johnny”, “My dearest John”, or simply “Darling”), a serviceman receiving a note beginning with a curt “Dear John” would instantly be aware of the letter’s purpose.