Once World War II ended, the Allies occupied the continent, giving some countries back to their pre-war leaders or creating new governments, before funding their nations' economic recovery.

Many Germans back then blamed their country's post-war economic collapse on the treaty's conditions and these resentments contributed to the political instability, which made it possible for Adolf Hitler and his Nazi Party to come to power.

[32][33] From 1919 to 1921, Italian fascist Benito Mussolini grew a base of supporters who wanted him to deal with Italy's political and economic crises, which involved civil conflict over the growth of socialism in the country.

In 1922, during a controversial general strike by a weakened trade unionist movement, Mussolini and his followers seized power in Rome and installed him as the Prime Minister of Italy to run the country alongside the pre-existing monarchy of King Victor Emmanuel III.

[34] Italy, Germany, and Imperial Japan — led by Emperor Hirohito and Prime Minister Hideki Tojo — increasingly allied with each other, and during World War II they would be known as the Axis powers.

[53] In 1938, German chemists Otto Hahn and Fritz Strassman, discovered nuclear fission, or the release of large amounts energy after the "nucleus of an atom splits into two or more smaller nuclei".

[45] To ensure Germany would not face resistance from the Soviet Union during an invasion, the two countries signed an agreement to neutrality named the Molotov-Ribbentrop Pact after secret negotiations from 23 to 24 August.

The Allied countries, motivated by their victory in World War I, had generally not worked to produce significant amounts of newer weapons and military equipment afterwards, feeling confident in what they already had, while Germany did the opposite since remilitarising in 1935.

The Allies in 1939 were "together superior in industrial resources, population, and military manpower", but German weapons, equipment, training and logistics made the Wehrmacht the most powerful army in the world.

Based on these observations, the Soviets began building railway spur tracks leading west toward Finnish wilderness, in particular toward Kuusamo, Suomussalmi, Kuhmo, and Lieksa.

Vidkun Quisling, of the Nasjonal Samling fascist party, proclaimed a new government on the evening of 9 April 1940, and he became the Prime Minister of Norway under Germany's administration during the war.

Chamberlain had been criticised for the failure of the Norwegian campaign, and Churchill had become the Labour Party's choice for leading the nation – even if they disliked his anti-socialist beliefs – because of his willingness to fight Germany.

A coalition government was formed, led by a war cabinet of Churchill, Chamberlain, the conservative Lord Halifax, and the Labour members Clement Attlee and Arthur Greenwood.

In March 1941, the U.S. Congress passed the Lend-Lease Act, which allowed the U.S. to send large amounts of aid: it ranged from "tanks, aircraft, ships, weapons and road building supplies to clothing, chemicals and food."

Hitler and the Nazi High Command were convinced that by October, Germany would have taken the entirety of European Russia and the Soviet regime would collapse after losing support domestically.

For this purpose, Germany constructed three extermination camps, Bełżec, Sobibór, and Treblinka, and the Reich Security Main Office lead by Heydrich deported Polish Jews to these locations.

The next day, the Japanese Ambassador Hiroshi Oshima went to German Foreign Minister Joachim von Ribbentrop, asking him to finally have Germany declare war on the United States.

The objective was to "sink, burn and destroy any enemy shipping found in the convoy assembly anchorage, and to put out of action the garrison (kill or capture) and the German installations in the port including the fish factories."

If it was destroyed, then the powerful German battleship Tirpitz — which had been defending Norway – could not be repaired at St Nazaire during potential future involvement in the Battle of the Atlantic, instead having to take trips to two other docks, during which the British could attack her easier.

The British faced heavy losses, with 169 of them killed and 200 taken prisoner, but they greatly hurt the German navy, forcing Germany to divert troops from elsewhere to defend their Atlantic operations.

The Manhattan Project was led by physicist J. Robert Oppenheimer, and was worked on mainly in Hanford, Washington; Los Alamos, New Mexico; and Oak Ridge, Tennessee.

5,000 British, American, and Canadian engaged in paratrooper landings and amphibious attacks on the fortified port of Dieppe in a move designed to divert German attention away from the Eastern Front at Stalin's request.

On 30 April, the British submarine HMS Seraph dropped his body off the coast of Spain, intending for him to be discovered by the Axis, who would find a note on his person that mentioned the Allies were planning to invade Greece and the island of Sardinia, launching a small attack on Sicily as a feint.

In retaliation, in autumn 1943, Heinrich Himmler began Operation Harvest Festival, the killing of the 45,000 Jewish prisoners that remained in forced labour in the Lublin District of occupied Poland.

For a week, they faced the German 16th Panzer Division led by Field Marshal Albert Kesselring, who were outnumbered but gave more resistance than expected because they had been preparing since the resignation of Mussolini.

In Operation Neptune, they invaded five different beaches in the French region of Normandy - nicknamed (from west to east) Utah, Omaha, Gold, Juno, and Sword - and established a beachhead.

Major General Fritz Beyerlein, commander of the Panzer Lehr division, noticed the movement and sent troops to the village of Tilly-sur-Seulles in between the Brits and Caen to slow the advance.

[165][166][167] 20 to 25 February is known as the "Big Week" because of Operation Argument, in which Allied air forces took off from southern Italy to perform a series of bombing raids on German industrial targets such as aircraft factories.

[181] The next day, at the Battle for Castle Itter in Tirol, renegade German troops and Americans allied to stop the Waffen-SS from assaulting a stronghold filled with French politicians held as prisoner.

Harry Truman forbade the recruiting of former Nazi officials or sympathizers, but the operation's leading organisations – the Joint Intelligence Objectives Agency and the Office of Strategic Services — ignored the stipulation, "eliminating or whitewashing incriminating evidence of possible war crimes from the scientists’ records".