General employment of toxic munitions in World War I made it necessary for the United States as a belligerent to protect its soldiers against gas attack, and to furnish means for conducting gas warfare. The postwar revulsion against the use of gas in no way guaranteed that it would not be used in another war; and to maintain readiness for gas warfare, Congress therefore authorized the retention of the Chemical Warfare Service as a small but important part of the Army organization. Between world wars, officers of the Chemical Warfare Service anticipated that in another conflict the Service would again be principally concerned with gas warfare, and they concentrated on defense and retaliation against it. The almost equal preparedness of the United States and other nations for gas warfare acted during World War II as the principal deterrent to the uses of gas. That it was not used has obscured the very large and vital effort that preparations for gas warfare required at home and overseas. This effort involved large numbers of American scientists and the American chemical industry as well as the Chemical Warfare Service, and served not only the Army but also the other armed forces of the United States and those of Allied nations. And in World War II the Chemical Warfare Service and its civilian collaborators came up with some new major weapons, notably the 4.2-inch mortar, generators for large-area smoke screening, flame throwers, and incendiary and flame bombs. The Service acquired in addition an entirely new mission, that of preparing the nation against the hazards of biological attack. In fulfilling its responsibilities the Chemical Warfare Service during the war compiled a record of achievement that readers of this book both in and out of the Army, will find instructive.