At the end of the American cavalry era, the Army disposed of its horses by machine gunning them to death. In our war, the dogs were treated the same way. Only it was done in a more “humane” manner. Some excess dogs were reassigned to other bases but most were killed. The US Military has pledged never to dispose of military working dogs in such a manner again. A change in the law now allows  military dogs to be retired after their service is completed. 

Please Read  Death of a Warrior