During World War II, Japan used chemical weapons during the Second Sino-Japanese War, but chemical weapons were not used on a large scale in the European theater or in conflict between the United States and Japan. After the war, though, chemical weapons were used in several conflicts, most notably in the Yemeni Civil War and Iran–Iraq War. During this period, the United States and the Soviet Union also continued to refine their chemical arsenals, developing new agents such as VX and binary chemical weapons.