In August 1914, these forces combined to start World War I, shattering the dreams of a new future and marking the beginning of Europe's decline in the twentieth century. World War I destroyed the Austrian-Hungarian Empire, the German Empire, the Ottoman Empire, and the Russian Empire. World War I, also known as the Great War, contributed to the decline of France and Great Britain as their economies and population were decimated. The soldiers who fought in the trenches of France and Germany became part of a lost generation who had little hope for the future.

World War I was a different type of war because it was fought on the European home front as well as on the battlefield. The civilian population was forced to make sacrifices, such as meatless Monday, and women entered the workforce to replace men who had gone to battle. Industrialization and technology also changed the methods of warfare. Nations developed more destructive weapons, such as mounted machine guns and poison gas, which resulted in the death of millions.

World War I also was a turning point in history. By 1919, the world of 1914 lay in ruins and the map of Europe had changed to reflect the rise of national states in Czechoslovakia and Yugoslavia. World War I ended the Russian Empire and led to the rise of Communism. Fascism in Italy and Nazism in German developed in opposition to Communism as well as dissatisfaction with the Versailles Peace Treaty.

The discontent of Germany and Italy, Russia's exclusion from the peace talks, and the resentment over the establishment of a Polish nation, as well as Japan's protesting the refusal of Western nations to recognize their claim over China all lead to anger and bitterness for the next 20 years. These feelings contributed to World War II in 1939.