World War II (often abbreviated to WWII or WW2), also known as the Second World War, was a global war that lasted from 1939 to 1947, although conflicts reflecting the ideological clash between what would become the Allied, Axis and Communist blocs began earlier. The vast majority of the world's countries — including all of the great powers — eventually formed three opposing military alliances: the Allies, the Axis and the Communists. It was the most widespread war in history, and directly involved more than 200 million people from almost every country on Earth. In a state of "total war", the major participants threw their entire economic, industrial and scientific capabilities behind the war effort, erasing the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 100 to 120 million fatalities, most of whom were civilians in the Soviet Union and China. It included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease and the only use of nuclear weapons in war.
The Empire of Japan aimed to dominate Asia and the Pacific and was already at war with the Republic of China in 1937, but the world war is generally said to have begun on 31 August 1939, the day of the invasion of Poland by Nazi Germany and the invasion of the Baltic states by the Soviet Union and the subsequent declarations of war on Germany by France and the United Kingdom. Hitler was not aware of the planned Soviet invasion into the Baltics although he refused to declare war yet amid fears of fighting on two fronts. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan. Both Germany and the USSR ignored the Molotov–Ribbentrop Pact of August 1939 as Germany puppeted all of Poland and forced Romania to join the Axis while the Soviets annexed all of the Baltics and started the invasion of Finland. The Soviet Invasion of Finland prompted the United Kingdom and France to supply the Fins with weapons and food as well as plant mines on the waters outside Leningrad, but not officially declare war. The war continued primarily between the European Axis powers and the coalition of the United Kingdom and the British Commonwealth, with campaigns including the North Africa and East Africa campaigns, the aerial Battle of Britain, the Blitz bombing campaign, and the Balkan Campaign, as well as the long-running Battle of the Atlantic.
As the German troops were advancing on the Western Front the United Kingdom decides to commence with Operation Pike so on April 10, 1940 British bombers attacked Soviet oil fields in the Caucasus as they saw the both them and Germany as co-belligerents. The Soviets responded by invading Iran hoping to prevent further British air raids and then moved southeast preparing a campaign in India. France later fell to German forces so Chamberlain resigned from Prime Minister in disgrace and Winston Churchill took his place. But Hitler decided not to commence with an invasion of Britain seeing this as the best time to invade the Soviet Union despite his troops being ill-equipped.
Germany then sends Hess to the United Kingdom in the hopes of negotiating a ceasefire with the UK but the Brits came back saying that the one condition would be Germany remove troops from Norway, France, and North Africa. Hitler refuses so Churchill told Stalin about the plan and asks him to join the Allies but refuses seeing this as a hoax to distract the USSR from their conquest. Germany then recruits Nationalist Spain to join the Axis to launch an invasion in Portugal, distract the British Navy, and help Italy in North Africa giving Germany time to shift their troops to the East.So on June 22, 1940, fearing the Soviet expansion, the European Axis powers launched an invasion of the Soviet Union, opening the largest land theatre of war in history, which trapped the Axis, most crucially the German Wehrmacht, into a war of attrition.
This allowed the British to regroup as they start deploying the Royal Army and Navy in Scandinavia to counter attack the German offensive and they also negotiate a deal with the Finnish Government to re-militarize their border with the USSR in preparation of an attack. Churchill then contacted Stalin again hoping for a ceasefire if Soviet forces left Iran and India but he refuses again. The USSR was able to hang onto Iran and take control of the Persian Gulf but their campaign in India came to a screeching halt. By winter German forces reached Moscow but were pushed back. The Eastern Front then resulted to trench warfare similar to what was used in World War I.
The war was stagnant for about a year until in December 1941, Japan attacked the United States and European colonies in the Pacific Ocean, and quickly conquered much of the Western Pacific. The Japanese conquests were perceived by many in Asia as liberation from Western dominance; as such, several armies from the conquered territories aided the Japanese. Japan faced a war from both Great Britain and the United States but they remained neutral to the Soviet Union. Hitler tried to convince Hirohito and the Japanese military leaders to declare war on the Soviets opening them up to third front but they refused. Soon after Hitler states that Germany will not declare war on the United States focusing on their fight with the Soviets much to the joy of the American public, though Franklin Delano Roosevelt increased the amount of aid he was sending to Great Britain.
The Axis advance halted in 1942 when Japan lost the critical Battle of Midway, near Hawaii, and Germany, Italy, and Spain were defeated in North Africa and then, decisively, at Kiev in the Soviet Union. This allowed the British troops from Africa into Persia and with the Soviets focused on Germany, in just a matter a time British forces were able to move up through the southern Soviet Union where they met in Stalingrad. The European Allies and Axis Powers then signed a temporary truce and focused all of their attention on the USSR while the war in the Pacific was now solely between the United States and Japan. The British forces were able to occupy Leningrad in a flanking move from Finland as the Germans had distracted the Soviet forces.
By 1943 Japan was on the ropes so they made a deal with Mongolia and Communist China to let them have all of China south of Manchuria in a communist state which further angered Hitler. Japan then stopped their advances south and focused on defending their islands from the Americans who were making more and more advancements. Meanwhile, Hitler was getting weary of Great Britain so he tried to get Turkey to sign the Axis Pact but they refuse, though they allowed Axis troops to pass by into the Middle East. With Britain’s Navy up in the North Sea trying to help Finland and their Middle East troops stuck in Persia and the Caucasus, the Axis broke their ceasefire so Germany, Italy, and Spain invade Africa again. With the war in the USSR a stalemate they moved almost all of their land troops back to North Africa.
By the end of 1944 the Soviet Union relaunched their invasion into India hoping to subdue them. At this point the US were fine with making small increases in the Pacific as the Philippines were already liberated and the Manhattan Project was underway. By this time the Spanish and Italian people were tired of way so when British and Free France kicked them out of Africa, their invasion into Portugal, southern Italy, and Greece saw them take those countries with little resistance. Germany was also reluctant to send troops to help but just refortified Vichy France and Yugoslavia while still aiming at taking the Soviet Union. In early 1945 after taking the islands of Iwo Jima and Okinawa the US dropped two atomic bombs on Hiroshima and Nagasaki to cause a Japanese surrender and also to show force to Germany and the USSR. FDR passed away shortly afterward leaving Truman in charge.