As a result of the United States entry into World War I that year, there was strong pressure to support the war effort. Urey had been raised in a religious sect that opposed war. One of his professors suggested that he support the wartime effort by working as a chemist. Urey took a job with the Barrett Chemical Company in Philadelphia, making TNT, rather than joining the army as a soldier. After the war, he returned to the University of Montana as an instructor in chemistry.