In 1964, the U.S. military began to use the tear gas CS in Vietnam. Here, a Marine has just thrown a CS grenade into a tunnel to flush out the enemy and make the tunnel unusable. Military planes also dispersed CS against enemy forces. CS is incapacitating but usually not lethal; nonetheless, its use evoked criticism at home and abroad, including condemnation by the UN. Marine: James H. Pickerell/Stock Connection; Gas: U.S. Army Ordinance Museum/Aberdeen Proving Grounds.