Conscientious objectors. The United States government wanted as many men as they could get to join the force voluntarily. However, a law was passed in 1916 establishing that men had to register regardless of their will. Conscientious objectors were those men who were refusing to join the forces for moral or religious reasons. There were about 16,000 of them who were either sent to prison or allowed to do non-military work (like farming or as strecher-bearers on the battlefields) since their conscience didn't allow them to kill.
