The Allies' economy and population compared with the Axis' lessened with the early Axis victories. However, this was no longer the case after the United States and Soviet Union joined the Allies in 1941. The Allies were able to have a higher production level compared with the Axis because the Allies had more natural resources. Also, Germany and Japan did not plan for a long war and had no ability to do so. Both tried to improve their economies by using slave laborers.