A letter received by an enlisted man from his girlfriend or wife back home who promised to wait for him. The letter usually goes something like this:

Dear John,

     Blah blah blah blah... It breaks my heart to tell you
this... blah blah blah blah blah I met a nice man named
Carlos at the church picnic a few months ago... blah
blah blah blah... we've fallen in love... blah blah blah
blah... I'm sorry but I just couldn't wait for you... blah 
blah blah blah.

     - Mary
These letters usually don't contain the lines "I'm a callous bitch" or "I needed what I needed when I needed it", probably because such statements would be redundant. The "Dear John" Letter was much more prevalent and more sinister during wartime, when soldiers were unable to avoid conscription and could not return home during leave. There is an urban legend about the Dear John Letter, wherein Iraqis tried to play on soldiers' fears during Desert Storm, but instead came up with broadcasts like "While you are sitting in the desert, Bart Simpson is sleeping with your wives!"

The best way to combat a "Dear John" Letter is to collect pictures of the sisters, cousins, and girlfriends of other members of your platoon, squadron, etc., mix her picture in with them, and send them back home with a letter than reads:

Dear Mary,

     I forget which one is you.  Please remove your 
picture and send the rest back to me.  Thank you.

     - John