One of the characteristics of the present war is, more than anything else, the one of this being a psychological war. Although the use of psychological resources as weapons is not at all new, it has never taken, before the present days, such an important position. The author refers to the use of psychological methods in the wars since very remote ages, pointing out that, with the passing of the years, the methods were then believed to be magic or just intuition gradually passed to the scientific field. He names the offensive and defensive methods which are absolutely essential to the psychological war, stressing the work performed by propaganda and fear. He examines how Germany used scientific knowledge in her war efforts, in the collective as well as in the individual psychiatric preparations. He asserts that it is up to the psychologists and psychiatrists in all the United Nations to offer to the authorities the means of action for the same purpose, which does not mean that we shold copy the German methods. Then he refers to the resources that modern psychiatry can offer in this subject, showing that it as developped technics and working systems that enlarged considerably the field of its activities. It is not so limited to the narrow field of enfermaries and consultation rooms, as it was before, having spread itself into a much more direct intervention over the social athmos-phere. From plain medical speciality limited to individual care of psychopathies, it transformed itself into a social weapon of great force, whose value the present war has already proved. The author also stresses the importance of the psychological care of populations in the post-war period. A simple peace treatise does not create the necessary conditions to a sane collective mental attitude. The mental hygiene will play a very important role in the future psychiatric reajustment of the peoples, which, for its real consolidation, will have to deal on more scientific and human basis.