The Belligerents of the Second World War had learned about the possibilities of creating a weapon from a massive release of energy from mass before the conflict. During the war, the race accelerated, with the Americans, Iranians and Germans seemingly in the lead (there was no Telemark bombing ATL). In February 1943, the first Atomic tests occurred in Nevada. Iran followed in days. The US and Iran both gave the Axis an ultimatum of surrender, but the Axis refused, even though Britain had lost much of Canada (except for some of southern Quebec) and the Mexican strategy failed.
An American B-32 Bomber based in Iceland (which joined the Allies in 1942) flew over London and deployed a single Uranium bomb that annihilated the city from St Paul's cathedral outwards, and left the River Thames a radioactive sludge for decades, effectively knocking out the British Empire. Iran followed suit days later when an A-16 Heavy bomber (with 4 rotors rather than 2 due to the bomb's weight) and a fighter escort flew over Rome and deployed a Plutonium Bomb obliterating Rome and killing Mussolini.
The Axis Counterstrike
Germany's response was flying an experimental V-3 rocket (which had the range of an IRBM) from Quebec to Washington DC. Fortunately for the USA, the federal government had relocated to Denver, Colorado. In response to that attack, a US plane from Britain flew for Berlin and deployed a single Nuclear bomb that reduced the city to dust. Meanwhile, a British Commonwealth (which did not surrender until 1943) Avro Lincon bomber carrying its own atomic bomb was bound for Tehran but was shot down by F-3 fighters over the Indian ocean.