The main factor in shaping the lines of modern Europe after World War I was the Treaty of Versailles and other peace settlements. The treaty, signed in 1919, aimed to address the consequences of the war but imposed harsh reparations on Germany and redrew national boundaries based on the principles of self-determination. It led to the creation of new countries, such as Czechoslovakia and Yugoslavia, while dismantling empires like Austria-Hungary and the Ottoman Empire. The treaty's punitive measures and resulting instability contributed to the rise of extremist movements, ultimately influencing the outbreak of World War II.