An Atomic Great War

From an engineering perspective it seems possible for nuclear weapons to have been built in the 1910s, provided the necessary scientific knowledge was provided. Shaped charges were tested as early as 1894, and exploding-bridgewire detonator technology was developed during the Manhattan Project. By the 1920s precision machining was possible with items such as aircraft engine pistons for Liberty engines and gearing systems for naval turbines. Avoiding sparks might be an issue though.

Acquiring uranium and producing enriched uranium and/or plutonium could be a challenge, although the lack of Teflon might not be an issue if some of the less efficient nuclear enrichment processes are used. Using those less efficient processes would increase the already large amount of electricity required for enrichment though, possibly making it prohibitive, although plutonium production reactors could be used to generate electricity to help meet some of the demand.

Nuclear weapons could be used as breakthrough charges in trench warfare, similar to the huge explosive charges used at the Battle of the Somme and the 1917 Battle of the Messines. It would require careful placement to maximize the crater's diameter instead of depth, but massive holes could be made in enemy lines for troops to advance through (although they would obviously suffer severe radiation sickness).

Navies could make use of them as powerful sea mines, heavy torpedoes, and possibly nuclear fireships. There were many large torpedoes in World War I, including those of around 62 cm, and Germany had designs for 70 cm and 75 cm torpedo designs that would have been large enough to carry the 72 cm diameter Little Boy bomb. Unlike with a conventional torpedo, a nuclear torpedo detonation within a few hundred meters will at the very least cause a mission kill, if not sink the ship entirely. Even ships that survive the blast would be contaminated with lethal amounts of radiation from the base surge which is impossible to decontaminate. While even the British Empire would only be able to afford a few nuclear weapons, they would represent the ultimate threat to high sea fleets and make the lowly torpedo boat (or even a fireship) a very dangerous weapon once again.

It might even be possible to deploy them from the air using a super-zeppelin. They could carry several tons of payload, although a parachute would probably be required to slow down the bomb enough for the zeppelin to escape the blast.

 

It was possible to control ships by radio by the early 1920s, if not earlier. USS Iowa was converted to a radio controlled target ship in 1921, while HMS Agamemnon was converted to a radio controlled target ship in 1923. That offers a possible way to control a nuclear fireship or torpedo.

 

Atomic fireships and large torpedoes would be some of the more feasible ways to deploy a nuclear weapon before the development of more advanced aircraft.

 

There were some large aircraft that might have been usable for deploying nuclear weapons though. In the 1930s the Kalinin K-7 was designed to carry 9,600 kilograms (21,200 pounds) of bombs, but its slow speed and low altitude would have been a major issue. Perhaps a parachute retarded bomb could have been used though.

Here are some photos showing just how large the K-7 was (source):