The Nazis considered most Slavs to be non-Aryan "Untermenschen". The Nazi Party's chief racial theorist, Alfred Rosenberg, adopted the term from Klansman Lothrop Stoddard's 1922 book "The Revolt Against Civilization: The Menace of the Under-man". In the secret plan Generalplan Ost ("Master Plan East") the Nazis resolved to expel, enslave, or exterminate most Slavic people to provide "living space" for Germans, but Nazi policy towards Slavs changed during World War II due to manpower shortages which necessitated limited Slavic participation in the Waffen-SS. Significant war crimes were committed against Slavs, particularly Poles, and Soviet POWs had a far higher mortality rate than their American and British counterparts due to deliberate neglect and mistreatment. Between June 1941 and January 1942, the Nazis killed an estimated 2.8 million Red Army POWs, whom they viewed as "subhuman".