Oil

Ever since the dying days of the eighteenth century, power and industry have been driven by coal. Factories, ships, and trains have all relied on it. Even the cookers of domestic kitchens have been powered by the gases that can be extracted from it. However, coal also has a rival, one with the power to change the world yet again.
From black rock to black liquid. Oil, in it's crude form, can be found across the world. From the Caucasus, to the Danube, to Mesopotamia, Indonesia, and Texas. Anywhere this dark gold can be found will prove to be immensely important for those who can control them. Everything from handheld paraffin lamps to the largest ships afloat can be powered by oil. In many cases, can only be powered by oil. Coal still burns, and still turns turbines, flywheels, and pistons. Yet only oil can power a car, or an aeroplane, and doesn't leave behind clinker or soot of the like that coal does. Quite minor nations have now found themselves major players because of what lies underneath their mountains and deserts. Just as major ones have found themselves at the mercy of these lesser peoples if they wish to remain ascendant.
No power better shows this than Germany. The vast coal seams of the Ruhr valley were essential to winning the great war. However, they have no significant oil fields of their own. The nearest lie in Romania and Russia, neither of which have any interest in keeping Europe's newest empire in mastery of the continent. The possibility of oil under the north sea offers interesting potential. Yet that would require confrontation with Britain, and even then, the technology to exploit, or even confirm, such a resource, does not yet exist. Yet with the future always approaching, what else can be done?
A factory powered by oil burns cleaner than coal, good for the health of the workers and locals; a ship, or train, that burns oil in its boilers will go faster and further than any equivalent; and coal simply cannot be used to power internal combustion engines as found in cars, trucks, tanks, or aeroplanes. How nations adapt, assuming they can, will define the balance of power for decades to come.