Where are works on chemical weapons and chemical warfare classed? That depends on what aspect is covered. We’ll touch on a few of the many aspects.
In international law, control of chemical weapons is classed in 341.735 Control of chemical and biological weapons, e.g., The chemical weapons convention: A commentary.
In military science, works about chemical warfare are classed in 358.34 Chemical warfare, e.g., Chemical warfare: A study in restraints. A general history of chemical warfare covering multiple wars (e.g., A history of chemical warfare) is classed in 358.3409 (built with 358.34 plus standard subdivision T1—09 History).
In military engineering, works about chemicals used as destructive agents are classed in 623.4592 Chemical agents. The upward hierarchy includes:
623.45 Ammunition and other destructive agents
623.459 Nonexplosive agents
At 623.459 is the class-here note: "Class here detection of nonexplosive agents"; that note has hierarchical force. Hence Detection technologies for chemical warfare agents and toxic vapors is classed in 623.4592.
What about history of chemical warfare in a specific war? The Manual note at 930-990 vs. 355.009, 355-359 begins with relevant advice:
930-990 vs. 355.009, 355-359 Military topics and war
Use 930-990 for works on military history that deal with the outcome of significant events in wars, e.g., the use of tanks on the Eastern Front and how their use affected various battles 940.54217. Use the history standard subdivisions in 355-359 for works emphasizing military history or topics without consideration of the general course of a war, e.g., changes in tank tactics during the course of World War II 358.18409044. If in doubt between 930-990 and 355-359, prefer 930-990.
Works like Hell in Flanders Fields: Canadians at the Second Battle of Ypres and Trial by gas: The British Army at the Second Battle of Ypres are much concerned with the outcome of the Second Battle of Ypres in World War I; hence those works are classed in 940.424 1915, western and Austro-Italian fronts. The upward hierarchy includes:
940.4 Military history of World War
940.42 Land campaigns and battles of 1914-1916
In contrast, Behind the gas mask: The U.S. Chemical Warfare Service in war and peace covers the years 1917-1929; it begins with World War I but goes beyond to treat continuing policy issues. It is classed in 358.34097309041 (built with 358.34 plus T1—09 plus T2—73 United States as instructed at T1—093-T1—099 Specific continents, countries, localities; extraterrestrial worlds plus notation 090 from the add table entry T1—093-T1—099:0901-0905 Historical periods plus 41 from T1—09041 1900-1919 as instructed in the add table entry; the historical period notation 41 for 1900-1919 is correct by the first-of-two rule.)
Remember the key advice in the Manual note: "If in doubt between 930-990 and 355-359, prefer 930-990."