in particular, by asserting a natural right of a strong nation to whatever the weak cannot hold by force. Some centrist, capitalist, world leaders, including Presidents of the United States and U.S. Generals, expressed support for an economic view of war. Marxist The Marxist theory of war is quasi-economic in that it states all modern wars are caused by competition for resources and markets between great (imperialist) powers, claiming these wars are a natural result of capitalism. Marxist economists Karl Kautsky, Rosa Luxemburg, Rudolf Hilferding and Vladimir Lenin theorized that imperialism was the result of capitalist countries needing new markets. Expansion of the means of production is only possible if there is a corresponding growth in consumer demand. Since the workers in a capitalist economy would be unable to fill the demand, producers must expand into non-capitalist markets to find consumers for their goods, hence driving imperialism. Demographic Demographic theories can be grouped into two classes, Malthusian and youth bulge theories: Malthusian Malthusian theories see expanding population and scarce resources as a source of violent conflict. Pope Urban II in 1095, on the eve of the First Crusade, advocating Crusade as a solution to European overpopulation, said: This is one of the earliest expressions of what has come to be called the Malthusian theory of war, in which wars are caused by expanding populations and limited resources. Thomas Malthus (1766–1834) wrote that populations always increase until they are limited by war, disease, or famine. The violent herder–farmer conflicts in Nigeria, Mali, Sudan and other countries in the Sahel region have been exacerbated by land degradation and population growth. Youth bulge According to Heinsohn, who proposed youth bulge theory in its most generalized form, a youth bulge occurs when 30 to 40 percent of the males of a nation belong to the "fighting age" cohorts from 15 to 29 years of age. It will follow periods with total fertility rates as high as 4–8 children per woman with a 15–29-year delay. Heinsohn saw both past "Christianist" European colonialism and imperialism, as well as today's Islamist civil unrest and terrorism as results of high birth rates producing youth bulges. Among prominent historical events that have been attributed to youth bulges are the role played by the historically large youth cohorts in the rebellion and revolution waves of early modern Europe, including the French Revolution of 1789, and the effect of economic depression