World War One was caused by rising militarism, imperialism, and nationalism in Europe in the early 20th century which led countries to form alliances that pulled them into war when the Archduke of Austria was assassinated, starting a long war dominated by trench warfare on the Western and Eastern Fronts and involving the Ottoman Empire; after the war, the Treaty of Versailles and League of Nations were formed in an attempt to establish lasting peace and new political systems like the mandate territories.