The USSR rebuilt its rail system and industrialized with five-year plans. As a result, railroad freight grew about 20 times from 20 to 400 billion tonne-km by 1941. But then disaster struck again: World War II in 1941 when Nazi Germany invaded the Soviet Union. In the first year or so of the war, traffic plummeted to about half its prewar value. But then the USSR started restoring and constructing railroads during wartime so that by the end of the war about half of the lost traffic had been recovered. After the war was over it took a few more years to restore the railroads and get back to the pre-war level of traffic.