The La Fayette Escadrille was a French fighter squadron defending that besieged country in 1916. It was an experimental unit, made up of French commanders, French airplanes, French ground support, and American pilots. A wealthy American lieutenant had coins made for his fellow pilots, each coin contained the squadron's symbol. One pilot was shot down on the east side of the western front and was captured and relieved of all identification by the Germans except for that one coin. He escaped in the night and made it back to the Allied side of No Man's Land and was captured by the French as a spy, until they saw his coin and realized he was un pilote Américain.
This started an Air Force tradition of carrying a "challenge coin" where if you arrive at a (generally unofficial) Air Force function someone could challenge you for your coin. If you couldn't produce your coin the drink is on you, but if you could provide your coin, the drink is on them. Needless to say, carrying your "Challenge Coin" is an Air Force tradition, and it has spread to all other branches of the armed forces, and other groups where people gather to do things as a group such as police forces, fire departments, and biker gangs. And churches mentored by retired military folks too.
I have coins from different units and different functions. My collection contains a USAF coin, a 5th Bombardment Wing (Heavy) coin, an 80th Fighter Squadron (Juvats!!!) coin, several 429th Electronic Combat Squadron coins, one for Operation Southern Watch and one for Operation Provide Comfort. I would like one for the 81st TFS Wild Weasels (I was a weasel feeder before weasels were cool (again)) and a Buff Stuffer coin to add to my collection, but looking through the coin rack at Buckley AFB I found this one.