The outbreak of World War II marked a turning point for jazz. The swing-era jazz of the previous decade had challenged other popular music as being representative of the nation's culture, with big bands reaching the height of the style's success by the early 1940s; swing acts and big bands traveled with U.S. military overseas to Europe, where it also became popular. Stateside, however, the war presented difficulties for the big-band format: conscription shortened the number of musicians available; the military's need for shellac (commonly used for pressing gramophone records) limited record production; a shortage of rubber (also due to the war effort) discouraged bands from touring via road travel; and a demand by the musicians' union for a commercial recording ban limited music distribution between 1942 and 1944.