Humanity has experienced three major periods of war – 500Bc the Greek wars, 1600AD the Thirty Years War and 1900AD the Two World Wars. These were the most significant times of war in human history, as far as is known to Western science. IR scientists do not know much about other regions in the world. All of these major periods of war brought forth the classics in the field of IR (International Relations) and the main political inventions, for example Hobbes with the principle of sovereignty, or Kant with the ideas for global democratic organisation, or the invention of the discipline of IR and the UN. All of these periods of war have been preceded by sudden massive population growth. This article will sketch the evolution of war and politics over the longue duree. For this purpose, the history of war and the history of political thought will be discussed. As causes for the major wars, sudden massive population growth is identified. How the latter causes war is theorised in the paper, as far as possible. Population is still massively growing in the times of the 21st century, and should be kept in reasonable limits. Arguments for protection of childlessness will be presented at the end of this article.