Conservative military and diplomatic historian Mark Moyar claimed that Halberstam, along with fellow Vietnam journalists Neil Sheehan and Stanley Karnow, helped to bring about the 1963 South Vietnamese coup against President Diệm by sending negative information on Diệm to the U.S. government in news articles and in private, all because they decided Diệm was unhelpful in the war effort. Moyar claims that much of this information was false or misleading. Sheehan and Halberstam won Pulitzer Prizes for their work on the war; Karnow, for a work on American imperialism in the Philippines.