William Arkin is an American journalist specializing in national security. He is one of the two lead reporters for the Washington Post "Top Secret America" website on the United States intelligence community. He writes the Post's "Early Warning" blog. 
Affiliated with the Post since 1998, he has, over 30 years, written or co-written a number of books and major reports. A substantial amount of his work has focused on nuclear weapons with the National Resources Defense Council (NRDC), an organization favoring arms reduction, including the authoritative (if dated) U.S. Nuclear Forces and Capabilities.
Previously, he was the defense correspondent for the Los Angeles Times and a commentator for MSNBC. He has worked for Greenpeace, the Institute for Policy Studies, and Human Rights Watch. In general, he has been identified with the left.  Conservative commentators have often questioned his positions, suggesting he despises the military.
He is an adjunct professor at the School of Advanced Aerospace Studies of the U.S. Air Force, researching the role of airpower in the 2006 Israel-Hezbollah war. As a 2006-2007 Carr Center for Human Rights Fellow at the Kennedy School of Government, Harvard University, he workded with Sarah Sewall on the civilian cost of war.