World War II (WWII or WW2), called the Great Patriotic War in the Soviet Union, was a global war involving fighting in most of the world and most countries. Most countries fought in the years 1939–1945 but some started fighting in 1937. Most of the world's countries, including all the great powers, fought as part of two military alliances: the Allies and the Axis Powers. World War II was the largest and deadliest conflict in all of history. It involved more countries, cost more money, involved more people, and killed more people than any other war in history. Between 50 to 85 million people died. The majority were civilians. It included massacres, the deliberate genocide of the Holocaust, strategic bombing, starvation, disease, and the only use of nuclear weapons against civilians in history.
The two sides were the Allies (at first China, France and Britain, joined by the Soviet Union, United States and others) and the Axis (Germany, Italy and Japan). The war in Asia began when Japan invaded China on July 7, 1937. The war began in Europe when Germany invaded Poland on September 1, 1939. France and Britain reacted by declaring war on Germany. By 1941, much of Europe was under German control, including France. Only Britain remained fighting against the Axis in North Africa, the Mediterranean, and the Atlantic. Germany gave up plans to invade Britain after losing an airplane battle. In June 1941, Germany invaded the Soviet Union, starting the largest area of war in history. On December 7, 1941, Japan attacked the United States at Pearl Harbor and invaded British and French colonies in Asia, and the two wars became one.
The Japanese victories were stopped in 1942, and in that same year, the Soviets won the huge Battle of Stalingrad. After that, the Allies started to fight back from all sides. The Axis were forced back into the Soviet Union, lost North Africa, and, starting in 1943, were forced to defend Italy. In 1944, the Allies invaded France, and came into Germany from the west, while the Soviets came in from the east. Germany surrendered on 8 May 1945. Japan formally surrendered on September 2, 1945. The war ended with an Allied victory.
After the war, the United Nations was set up to develop support between countries and to prevent future wars. The Cold War among the major winners soon started, but they did not fight each other in an actual war. Decolonization of Asia and Africa, where those countries controlled by European countries were given their independence, happened as well. This was because European power was weakened from the war. Economic recovery and the political integration (the process of uniting countries) were among other results of the war.
The countries that joined the war were on one of two sides: the Axis and the Allies.
The Axis Powers at the start of the war were Germany, Italy, and Japan. There were many meetings to create an alliance between these countries. Finland, Slovakia, Romania, Bulgaria, Hungary, and Thailand joined the Axis later. As the war continued, some Axis countries changed to join the Allies instead, such as Italy.
The Allied Powers were the United Kingdom and some Commonwealth members, France, Poland, Yugoslavia, Greece, Belgium, and China at the start of the war. China had been fighting a civil war. In June 1941, Germany attacked the Soviet Union in Operation Barbarossa. In December 1941 came Japan's Attack on Pearl Harbor against the United States. These two large, powerful countries then joined the Allies.
World War I had greatly changed the way of diplomacy and politics in Asia, Europe, and Africa with the defeat of the Central Powers. Empires that sided with the Central Powers were destroyed. The Russian Empire, which did not side with the Central Powers, died as well. The war also changed the borders in Eastern Europe, with many new countries born. The war led to strong irredentism and revanchism. These senses were especially strong in Germany, which had no choice but to sign the Treaty of Versailles. The Germans also had 13% of their homeland area and all colonies taken away, and they had to pay back a very large sum of money to the Allies. The size of their army and navy was also limited, while its air force was banned.
In Italy, nationalists were unhappy with the outcome of the war, thinking that their country should have gained far more territory from the past agreement with the Allies. The fascist movement in the 1920s brought Mussolini to the leadership of the country. He promised to make Italy a great power by creating its colonial empire.
After the Kuomintang (KMT), the governing party of China, unified the country in the 1920s, the civil war between it and its past ally Communist Party of China began. In 1931, Japan used the Mukden Incident as a reason to take Manchuria and set up its puppet state, Manchukuo, while the League of Nations could not do anything to stop it. The Tanggu Truce, a ceasefire, was signed in 1933. In 1936, the KMT and the communists agreed to stop fighting against each other to fight Japan instead. In 1937, Japan started a Second Sino-Japanese War to take the rest of China.
After the German Empire was disestablished, the democratic Weimar Republic was set up. There were disagreements between the Germans which involved many political ideologies, ranging from nationalism to communism. The fascist movement in Germany rose because of the Great Depression. Adolf Hitler, leader of the Nazi Party, became the Chancellor in 1933. After the Reichstag fire, Hitler created a totalitarian state, where there is only one party by law. Hitler wanted to change the world order and quickly rebuilt the army, navy and air force, especially after Saarland was reunited in 1935. In March 1936, Hitler sent the army to Rhineland. The Spanish Civil War began in July 1936. The war ended with the nationalist victory, supported by Italy and Germany.
In March 1938, Germany sent its army into Austria, known as the Anschluss, which had only a little reaction from European countries. Shortly after that, the Allies agreed to give Sudetenland, part of Czechoslovakia, to Germany, so that Hitler would promise to stop taking more land. But the rest of the country was either forced to surrender or invaded by March 1939. The Allies now tried to stop him, by promising to help Poland if it was attacked. Just before the war, Germany and the Soviet Union signed a peace agreement, agreeing that they would not attack each other for ten years. In the secret part of it, they agreed to divide Eastern Europe between them.
World War II began on September 1, 1939, as Germany invaded Poland. On September 3, Britain, France, and the members of the Commonwealth declared war on Germany. They could not help Poland much and only sent a small French attack on Germany from the West. The Soviet Union invaded eastern Poland soon after Germany, on September 17. Finally, Poland was divided.
Germany then signed an agreement to work together with the Soviet Union. The Soviet Union forced the Baltic countries to allow it to keep Soviet soldiers in their countries. Finland did not accept the Soviet call for its land, so it was attacked in November 1939. With peace, the world war broke out. France and Britain thought that the Soviet Union might enter the war on the side of Germany and drove the Soviet Union out of the League of Nations.
After Poland was defeated, the "Phoney War" began in Western Europe. While British soldiers were sent to the Continent, there were no big battles fought between the two sides. Then, in April 1940, Germany decided to attack Norway and Denmark so that it would be safer to transport iron ore from Sweden. The British and French sent an army to disrupt the German occupation, but had to leave when Germany invaded France. Chamberlain was replaced by Churchill as Prime Minister of United Kingdom in May 1940 because the British were unhappy with his work.