Mustard gas or sulfur mustard is a chemical compound belonging to a family of cytotoxic and blister agents known as mustard agents. The name mustard gas is widely used, but it is technically incorrect: the substance, when dispersed, is often not actually in a vapor, but is instead in the form of a fine mist of liquid droplets.Dichlorodiethyl sulfide is a symmetric, short chain molecule, with a sulfur atom in the middle and chlorine atom at both ends, chemical formula C4H8Cl2S. In the wider sense, compounds with the structural element BC2H4X, where X is any leaving group and B is a Lewis base, are known as mustards. Such compounds can form cyclic -onium ions (sulfonium, ammonium, etc.) that are potent alkylating agents of nucleotides in DNA strands, which prevents cellular division, leading to programmed cell death. Alternatively, if cell death is not immediate, the damaged DNA can lead to the development of cancer. Oxidative stress is another pathology involved in mustard gas toxicity.

Mustard gas has a long history of being used as a blister agent in warfare and is one of the most well-studied of such agents. It can form large blisters on exposed skin and in the lungs, often resulting in prolonged illness ending in death. Sulfur mustards are viscous liquids at room temperature and have an odor resembling mustard plants, garlic, or horseradish, hence the name. When pure, they are colorless, but when used in impure forms, such as in warfare, they are usually yellow-brown.

As a chemical weapon, mustard gas was first used in World War I, and has been used in several armed conflicts since then, including the IranIraq War, resulting in more than 100,000 casualties. Today, sulfur-based and nitrogen-based mustard agents are regulated under Schedule 1 of the 1993 Chemical Weapons Convention, as substances with few uses other than in chemical warfare (though since then, mustard gas has been found to be useful in cancer chemotherapy). Mustard agents could be deployed by means of artillery shells, aerial bombs, rockets, or by spraying from aircraft.

The air raid on Bari (German: Luftangriff auf den Hafen von Bari, Italian: Bombardamento di Bari) was an air attack by German bombers on Allied forces and shipping in Bari, Italy, on 2 December 1943, during World War II. 105 German Junkers Ju 88 bombers of Luftflotte 2 achieved surprise and bombed shipping and personnel operating in support of the Allied Italian Campaign, sinking 27 cargo and transport ships, as well as a schooner, in Bari harbour.

The attack lasted a little more than an hour and put the port out of action until February 1944. The release of mustard gas from one of the wrecked cargo ships added to the loss of life. The British and US governments covered up the presence of mustard gas and its effects on victims of the raid.