Two World Wars and an economic depression dominated the first half of the 20th century. World War I was fought between 1914 and 1918. It started when Archduke Franz Ferdinand of Austria was assassinated by the Bosnian Serb Gavrilo Princip. Most European nations were drawn into the war, which was fought between the Entente Powers (France, Belgium, Serbia, Portugal, Russia, the United Kingdom, and later Italy, Greece, Romania, and the United States) and the Central Powers (Austria-Hungary, Germany, Bulgaria, and the Ottoman Empire). The War left around 40 million civilians and military dead. Over 60 million European soldiers were mobilised from 1914–1918.
Partly as a result of its defeat Russia was plunged into the Russian Revolution, which threw down the Tsarist monarchy and replaced it with the communist Soviet Union. Austria-Hungary and the Ottoman Empire collapsed and broke up into separate nations, and many other nations had their borders redrawn. The Treaty of Versailles, which officially ended World War I in 1919, was harsh towards Germany, upon whom it placed full responsibility for the war and imposed heavy sanctions.
Economic instability, caused in part by debts incurred in the First World War and 'loans' to Germany played havoc in Europe in the late 1920s and 1930s. This and the Wall Street Crash of 1929 brought about the worldwide Great Depression. Helped by the economic crisis, social instability and the threat of communism, fascist movements developed throughout Europe placing Adolf Hitler of Nazi Germany, Francisco Franco of Spain and Benito Mussolini of Italy in power.
By the end of World War II, the European economy had collapsed with 70% of the industrial infrastructure destroyed.
In 1933, Hitler became the leader of Germany and began to work towards his goal of building Greater Germany. Germany re-expanded and took back the Saarland and Rhineland in 1935 and 1936. In 1938, Austria became a part of Germany too, following the Anschluss. Later that year, Germany annexed the German Sudetenland, which had become a part of Czechoslovakia after the war. This move was highly contested by the other powers, but ultimately permitted in the hopes of avoiding war and appeasing Hitler.
Shortly afterwards, Poland and Hungary started to press for the annexation of parts of Czechoslovakia with Polish and Hungarian majorities. Hitler encouraged the Slovaks to do the same and in early 1939, the remainder of Czechoslovakia was split into the Protectorate of Bohemia and Moravia, controlled by Germany, and the Slovak Republic, while other smaller regions went to Poland and Hungary. With tensions mounting between Germany and Poland over the future of Danzig, the Germans turned to the Soviets, and signed the Molotov-Ribbentrop Pact. Germany invaded Poland on 1 September 1939, prompting France and the United Kingdom to declare war on Germany on 3 September, opening the European theatre of World War II. The Soviet invasion of Poland started on 17 September and Poland fell soon thereafter.
On 24 September, the Soviet Union attacked the Baltic countries and later, Finland. The British hoped to land at Narvik and send troops to aid Finland, but their primary objective in the landing was to encircle Germany and cut the Germans off from Scandinavian resources. Nevertheless, the Germans knew of Britain's plans and got to Narvik first, repulsing the attack. Around the same time, Germany moved troops into Denmark, which left no room for a front except for where the last war had been fought or by landing at sea. The Phoney War continued.
In May 1940, Germany attacked France through the Low Countries. France capitulated in June 1940. However, the British refused to negotiate peace terms with the Germans and the war continued. By August Germany began a bombing offensive on Britain, but failed to convince the Britons to give up. In 1941, Germany invaded the Soviet Union in the ultimately unsuccessful Operation Barbarossa. On 7 December 1941 Japan's attack on Pearl Harbor drew the United States into the conflict as allies of the British Empire and other allied forces.
After the staggering Battle of Stalingrad in 1943, the German offensive in the Soviet Union turned into a continual fallback. In 1944, British and American forces invaded France in the D-Day landings, opening a new front against Germany. Berlin finally fell in 1945, ending World War II in Europe.
The war was the largest and most destructive in human history, with 60 million dead across the world. More than 40 million people in Europe had lost their lives by the time World War II ended, including between 11 and 17 million people who perished during the Holocaust. The Soviet Union lost around 27 million people during the war, about half of all World War II casualties. By the end of World War II, Europe had more than 40 million refugees. Several post-war expulsions in Central and Eastern Europe displaced a total of about 20 million people.
The Schuman Declaration (9 May 1950) lead to the creation of the European Coal and Steel Community. It began the integration process which today comprises the European Union of 27 democratic countries in Europe.
World War I and especially World War II diminished the eminence of Western Europe in world affairs. After World War II the map of Europe was redrawn at the Yalta Conference and divided into two blocs, the Western countries and the communist Eastern bloc, separated by what was later called by Winston Churchill an "iron curtain". The United States and Western Europe established the NATO alliance and later the Soviet Union and Eastern Europe established the Warsaw Pact.
The two new superpowers, the United States and the Soviet Union, became locked in a fifty-year long Cold War, centred on nuclear proliferation. At the same time decolonisation, which had already started after World War I, gradually resulted in the independence of most of the European colonies in Asia and Africa. In the 1980s the reforms of Mikhail Gorbachev and the Solidarity movement in Poland accelerated the collapse of the Eastern bloc and the end of the Cold War. Germany was reunited, after the symbolic fall of the Berlin Wall in 1989, and the maps of Eastern Europe were redrawn once more. European integration also grew in the post-World War II years. The Treaty of Rome in 1957 established the European Economic Community between six Western European states with the goal of a unified economic policy and common market. In 1967 the EEC, European Coal and Steel Community and Euratom formed the European Community, which in 1993 became the European Union. The EU established a parliament, court and central bank and introduced the euro as a unified currency. In 2004 and 2007, Eastern European countries began joining, expanding the EU to its current size of 27 European countries, and once more making Europe a major economical and political centre of power.