Havana Syndrome is most likely caused by unknown types of directed energy weapons, either of the acoustic or electromagnetic variety. The technology for such weapons has been under development since at least the 1960s. The first covert attacks on U.S. diplomats may go back to that time when the phenomenon was called the “Moscow Signal.” Havana Syndrome: A Threat to National Security covers the history of anti-personnel directed-energy weapons (DEW) from the Soviet psychotronic weapons that were rumored to exist since the 1970s to the nonlethal weapons developed in the West in the 1980s onward. The book examines the publicly known details of Havana Syndrome and provides some analysis of the U.S. government investigations into Havana Syndrome with respect to what has been declassified. The most likely perpetrators are the Russian or Chinese governments, as some incidents seem to be related to their strategic interests, and as both would have the technology to carry out such sophisticated attacks. Havana Syndrome establishes a connection between the Anomalous Health Incidents and what NATO analysts have termed cognitive warfare to speculate about the true purpose of these mysterious attacks.