Volume 20 of the World War II Sea War series provides additions and corrections to the information originally provided in Volume 5 of the series. As in Volumes 18 and 19, the General sections have been expanded to help form the overall context of the war, especially how world leaders made decisions day-by-day. Much of Volume 20 expands on Imperial Japanese Navy and merchant ship movements as Japan prepared to bring the United States into the War. In particular, we provide additional data on the Japanese invasions of Malaya, Burma, Sumatra, Borneo, Java, Celebes, Solomon Islands, and New Guinea, which occurred during the first three months of 1942. To our knowledge, this is the first time this information has appeared in print in English. In addition to this expansion of Japanese data, Volume 20 includes new data on the Royal Indian Navy, German U-boats, and the Royal New Zealand Navy. This volume also expands on the Royal Navy’s actions off Norway.