This article deals with the question «Why did the First World War start?» Here will be discussed such topics as geopolitical interests of the countries which were involved in the war, international relations, the events which make the countries start the conflict, the preparation for it and international crises before the tragedy. The article deals with the policy of such countries as Serbia, Austria-Hungary, Germany, Russia, Great Britain and France. The reader will also find information about the assassination in Sarajevo, its influence on the war. World War I became the prologue to the 20th century. Without any doubt, it could be described as turning point in the history. It is impossible to understand the events of the 20th century which are discussive nowadays without the knowledge of World War I reasons. It led to the collapse of 4 empires, the creation of many new states including the USSR which was a unique one for that time, the situation where winners of the war should have built new system of international relations, but they did not succeed. New political movements appear which were ruinous for the peace; a war became more violent since then. War and ultimatums still exist in the world as tools of foreign policy. For example, not far from now D. Trump has been about to start a war with Iran which is one of the most important political powers in the Middle East or Ukraine supported by NATO have been acting aggressively for a long time. It could lead to a new global conflict with many victims of it or even to the extinction of the humankind. Modern people should remember about the consequences and try to prevent a new big war. If the actors of the international relations understand the reasons of World War I, they can avoid that traps and prevent new tragedy.