The Nazis persecuted numerous groups they deemed as threats to their vision of a Master Race, including Jews, the mentally ill, homosexuals, Roma people, Jehovah's Witnesses, and others. They implemented forced sterilization, imprisonment in concentration camps where many were experimented on or killed, and later began the Holocaust against Jews in particular. Historians disagree on whether Hitler explicitly planned the Holocaust from the beginning or if anti-Semitic policies escalated chaotically over time.