The bibliography of this subject does not readily admit of the conventional distinction between primary and secondary sources. It has seemed more natural to use the categories of ‘unpublished’ and ‘published’. ‘Published’ documents are those which have been made available to the general public: some of the ‘unpublished’ documents have been published to restricted classes of readers. The list of published works omits those which, though they may have furnished incidental quotations or references, do not bear directly on this military history or its strategic background.