http://www.youtube.com/watch?v=SeiqPdcxowA
The dog tag is a symbol of military tradition; it was used before as part of the military uniform. The image portrayed by the oblong metal discs hanging from the beaded metal chain is both masculine as well as iconic. It was used to represent the American soldiers in movies and television. The little metal discs, we're not intended to be icons but something a bit simpler than that.

Dog tags came into existence during the American Civil War; this was used by many
troops at that time as a means of identification. When they fall in battle -- dog tags are used for official identification. The original dog tags, also known as the field-expedient identification tag was often nothing more than a wooden disc adorned with the name of the soldier, worn around the neck using a bit of string.
http://www.youtube.com/watch?v=SeiqPdcxowA