Nuclear Steam

In a world where oil never existed, humanity perfected their steam engines and the electricity-producing gears attached to them, to the point that while they still used coal for the biggest electricity producing plants, they were able to make a version of car and plane engines which used steam pressure from the intense heat of a hydrogen flames running along a boiler between the cylinders. Eventually they discovered the power of the atom, and discovered that while using them for small engines was (at the time) impossible, they could harness its sheer heating capabilities in absolutely tiny quantities compared to even the best coal, and produce way more power for longer with a much cleaner impact on the environment. So after converting all of the coal power plants to nuclear, they began working on smaller and smaller nuclear steam boilers, until they finally were able to produce boilers for even the smallest car engine block giving the owners the greatest fuel longevity they could ever hope for. Unfortunately, it wasn't thirty years after this that the only discovered source of nuclear energy for them, uranium, began to (seemingly) grow scarce, leading to a planet-wide war between the nations; while about a dozen of the largest nations had joined one side or the other against each other, somehow managing an even 6v6, all of the others had gone every-nation-for-themself, attacking everyone who came near and hoarding what uranium they had stockpiled to themselves. This story takes place ten years into this war, and most of those smaller nations have fallen, leaving only five which have chosen to band together for survival as most of the other smaller have been destroyed by the Twelve; one of the Twelve have fallen on either side, reducing them to five one each side.