This chapter presents the main cause of internal and international peace. In order that a diagnosis of the preconditions of peace may be precise and fruitful, one can distinguish between the main or necessary cause of peace and its supplementary factors, both positive and negative. The principle of the main cause and supplementary factors is generally used in causal analysis throughout the natural sciences. Nearly all the advances in the natural sciences have involved the discovery of the main or necessary cause of a given phenomenon, and some of its supplementary factors. The theory of multiple causation is not applicable to sociocultural phenomena generally, or to the study of war and peace in particular. It is far more inadequate than the theory of a main cause and supplementary factors. The "multiple-factor" theory of war or peace looks nice on paper, especially when it is given a mathematical appearance.