Why has the United States decided to crack down on suspected Japanese war criminals 50 years after granting them immunity from prosecution? Japanese scratched their heads last week at the unexpected announcement that the U.S. Justice Department had included former members of an infamous bacterial warfare research unit on a watch list" of 16 suspected Japanese World War II war criminals prohibited from entering the United States. The United States has been aware of the identities of the Unit 731 leaders and of their gruesome experiments on human subjects since the end of the war. Details of Unit 731 atrocities have appeared in the Western and Japanese media for more than a decade. In secret laboratories in occupied China, Unit 731 researchers tested poison gas and biological weapons on prisoners; froze and defrosted victims' limbs to study frostbite; and vivisected humans without anesthetic. After the war, the United States concluded that the results of these experiments were "of the highest intelligence value." Fearful that those results would fall into Soviet hands, the U.S. occupation authorities gave the head of Japan's bacterial warfare program, Dr. Shiro Ishii, and his colleagues immunity from prosecution ... in exchange for their secret data. Many of Ishii's colleagues went on to distinguished careers in postwar Japan, holding posts in the National Institute of Health, serving as medical school deans and laboratory heads.