The Final Months of the War With Japan: Signals Intelligence, US Invasion Planning, and the A-Bomb Decision

Dedication

The author of this monograph, Douglas J. MacEachin passed away on January 20, 2021. Born on July 24, 1937, he served in the Marines and had a long and fruitful career in intelligence analysis at CIA.

Foreword

This monograph was produced under the auspices of CIA’s Center for the Study of Intelligence and the Harvard University program for Studies of Intelligence and Policy. The idea was to examine the role of signals intelligence* in US military planning during the final stages of the war with Japan in 1945–particularly its contribution to planning for an Allied  invasion of the Japanese homeland.

This study was not intended as an argument for or against the use of the atomic bomb against Japan. Obviously, the importance of the bomb in concluding the war was of such magnitude that it is not  plausible to  examine intelligence related to invasion planning without addressing the question of  whether and to what extent that same intelligence might have influenced the decision to drop the  bomb. It also is not plausible to argue that the military calculus concerning an invasion of Japan does  not bear directly on evaluations of the bomb decision. Nonetheless, the debates and historical studies supporting or condemning the use of the bomb involve factors that go well beyond the scope of this monograph.

The study’s basic objective is not to pass judgment on the decisions that were made, but rather to examine the intelligence that was available at the time and to weigh the role this intelligence played or might have played in the deliberations on an invasion.

The author wishes to express his appreciation to those who reviewed drafts of this study and provided constructive comments–particularly military historian Edward Drea.

*In modern intelligence parlance, the term signals intelligence, or SIGINT, is often used to refer to a broad range of intercepted communications.

Note to Readers:

This publication appears in two downloadable PDFs. The first is the narrative monograph, containing discussions of the phases of US deliberations about military action to end the war and the data that underpinned it. This section also contains lists of resources on this topic and summaries of the documents contained in the second PDF.  Both PDFs are bookmarked.