The way Civ handled Strategic Resources was always stupid, both from a historical roleplay and game mechanics standpoint, it’s why No Iron = Restart became a meme.
Prussia and then Germany are the best examples of how this is also a fail from a historical roleplay perspective. Prussia somehow managed to get through the 7 years war, the “Potatoe War” period and the Napoleonic wars period despite having in Civ terms no horses, iron, or nitre.
Germany is even worse. The only strategic resource they would have had is coal, and they somehow managed to wage war in the Combustion Era for 6 years while dramatically expanding the number of planes, tanks, Uboats engaged in combat.
When their wartime economy imploded and they started having serious issues with planes without fuel and Panzers with no lubricating oil it didn’t happen till basically the end of the war, and the biggest factor was remorseless American bombing that levelled their industrial base and transport infrastructure.
The Luftwaffe didn’t disappear from the skies for lack of fuel. It got remorselessly attritioned out of existence by the Americans. It’s the same story for their ground forces, they got beaten to death by a huge American army, and British and Russian armies underwritten by the Americans.
Prussia and Germany in Civ VII's "Modern Age" are actually examples of insufficient in-game flexibility in Resource sources.
By the 18th century Nobody relied on a natural 'Nitre' resource, the resource was manufactured in Nitraries and had been for several centuries throughout Europe.
Prussia at the beginning of the 18th century set up stud farms (subsidized) to 'produce' horses for the military, and was also able to buy horses in quantity from Poland and other states throughout the century. - And by the way, the greatest need for horses was not by the cavalry, but by the artillery, which required up to 8 horses to haul each gun plus another 18 - 24 horses to haul the ammunition caisson/wagons for each gun - and unlike light cavalry mounts, the 'artillery horses' had to be substantial beasts: in the twentieth century, the German military classified as Artillery Horses the heaviest horses they acquired - "riding horses" were lighter and 'cart horses' the lightest and smallest.
And note that for the 18th century at least, Prussia was not at war with Sweden, which was her prime source for iron ore - a situation which repeated itself in WWII, when protecting the 'trade route' for the ore from Narvik down the Norwegian coast was one of the primary reasons why Germany attacked and occupied Norway in 1940.
The requirement for almost all natural resources was never much of an impediment given the relatively small quantities required before Industrialization. Once Krupp was casting steel cannon and supplying armies of millions rather than 10s of thousands, the requirements for iron ore, coal and alloy metals (nickle, cadmium, chromium, etc) required for specialized steels (armor plate the prime example) were in the hundreds or thousands of tons and supply sources became critical.
The Death of the Luftwaffe, by the way, was both from attrition in campaigns like Big Week in early 1944 but also because fuel supplies got so critical that they didn't have enough to train new pilots. Replacing an experienced pilot shot down by swarms of P-51s or La-7s with a new pilot who got less than 100 hours' flying time before going into battle simply accelerated losses to the breaking point, and some aircraft in the end could not be flown because they required experienced pilots to handle them and those were no longer available - the majority of the famous Me-262 jet fighters produced never flew because there were not enough pilots left with the experience to handle them: the early jets were not easy to handle and because they were new, pilots were learning as they went, not something a green pilots could do and survive.
