The early 20th century was plunged into catastrophic conflict. World War I erupted from a combination of militarism, alliances, imperialism, and nationalism (often summarized as MAIN), triggered by the assassination of Archduke Franz Ferdinand of Austria-Hungary in Sarajevo on June 28, 1914, by Gavrilo Princip, a Bosnian Serb nationalist. The war on the Western Front became defined by trench warfare, where opposing armies dug extensive trench systems and faced a bloody stalemate involving machine guns, artillery, barbed wire, and poison gas, producing massive casualties on both sides.
The war's outcome reshaped Europe and the world. The Treaty of Versailles (1919) imposed harsh terms on Germany, including territorial losses, military restrictions, acceptance of war guilt, and massive reparations. Its punitive nature contributed to economic hardship and the rise of Nazism. Meanwhile, the League of Nations, established in 1920 as the first international organization aimed at maintaining world peace, ultimately failed due to its lack of enforcement power, the absence of the United States, and its inability to prevent aggression. Far to the east, the Russian Revolution of 1917 unfolded in two phases: the February Revolution ended tsarist rule, while the October Revolution brought the Bolsheviks under Vladimir Lenin (1870–1924) to power, leading to the creation of the Soviet Union and the spread of communism.
The global economy then collapsed with the Great Depression (1929–1939), triggered by the U.S. stock market crash of 1929 and causing mass unemployment, poverty, and political instability worldwide. These conditions fueled the rise of fascism in the 1930s under Benito Mussolini in Italy and Adolf Hitler (1889–1945) in Germany, characterized by extreme nationalism, authoritarianism, militarism, and suppression of opposition. Hitler orchestrated the Holocaust, the systematic, state-sponsored genocide of approximately six million Jews, along with millions of others including Roma, disabled people, and political prisoners. World War II began through a combination of factors, including the punitive Treaty of Versailles, the Great Depression, fascist and militarist aggression, and appeasement policies. Allied victory came through major operations like D-Day (June 6, 1944), the largest amphibious military operation in history, and the atomic bombings of Hiroshima and Nagasaki on August 6 and 9, 1945, which killed over 200,000 people and led to Japan's surrender, ending the war but ushering in the nuclear age.
In response to these catastrophes, the United Nations was established on October 24, 1945, to maintain international peace and security, replacing the failed League of Nations with bodies like the Security Council and General Assembly. In 1948, the UN adopted the Universal Declaration of Human Rights, drafted under Eleanor Roosevelt's leadership, outlining fundamental rights and freedoms for all people regardless of nationality, race, or religion. Together, the two world wars shattered European dominance, gave rise to new ideological struggles, and created the institutional framework of the contemporary international order.