- Iron (from Sweden)
- Tungsten (from Portugal and Spain)
- Chromium (from Turkey)
What if the Allies used military means to cut off Germany from these sources?
The British and French planned to deprive Germany of iron ore from Sweden by sending an army to help Finland against the Red Army, but Sweden never gave permission for them to pass through before the Finns and Soviets agreed to a peace. Nevertheless, the British, French and Norwegian forces succeeded in capturing Narvik in 1940, cutting off one of the two ports used to ship Swedish iron ore to Germany (the other one being Luleå in Sweden, which was frozen off during the winter). In the OTL, the Allies withdrew from Narvik in June due to the collapse in France. What if they chose to stay and hold Narvik?
There was no overland connection between Narvik and the part of Norway under German control. The German fleet had been severely mauled, and the Allies controlled all the ports in Norway. And Narvik was out of range of the Luftwaffe. The British could have stayed, along with Norway's king, who in the OTL ordered the victorious Norwegian troops in Narvik to surrender. And there would have been nothing Germany could do about it.
In the OTL, Britain reached a deal with Turkey to buy up 100% of its chromium in 1939 through the end of 1942. Germany and Turkey signed a deal in October 1941 to resume Turkish shipments of chromium to Germany starting in 1943. The Allies put diplomatic pressure on Turkey that finally succeeded in 1944 in getting Turkey to stop selling chromium to Germany. Germany had bought large stocks of chromium from Turkey in the 1930s and supplemented it with chromium from the Balkans and Soviet deliveries prior to Barbarossa, and this allowed German industry to stay afloat through 1942, but the situation was becoming critical by 1943. Albert Speer wrote:
https://1997-2001.state.gov/www/regions ... turkey.pdfHence the element in shortest supply is chromium. This is especially grave since chromium is indispensable to a highly developed armaments industry. Should supplies from Turkey be cut off, the stockpile of chromium is sufficient only for 5.6 months. The manufacture of planes, tanks, motor vehicles, tank shells, U-boats, and almost the entire gamut of artillery would have to cease from one to three months after this deadline, since by then the reserves in the distributions channels would be used up.
Germany required 3,500 tons of iron ore per year, most of which came from German owned mines in Spain and Portugal, as discussed here: viewtopic.php?f=66&t=2569
Timing for invasions
The main question is, when could the Allies have invaded Sweden, Portugal and Turkey to deprive Germany of these metals? The most likely time would be summer 1943, after the liberation of North Africa was complete and the Allies had relatively free transit through the Mediterranean. Instead of invading Sicily, which served little strategic purpose, the Allies could have landed troops in Turkey and Spain, and also crossed over the border from Norway and occupied the Kiruna iron mines in Sweden. The Allies might have been able to move earlier, but they could definitely do so by July 1943.
What would be the effect? The simultaneous elimination of most of Germany's supply of high quality iron ore, chromium and tungsten would have devastated German industry. Germany could ration its existing stocks and find smaller amounts from other sources (mainly the Balkans), but Germany would also be losing its manganese and iron mines in the Ukraine by early 1944. And there would be nothing Germany could do to stop the Allies. Germany could not reach northern Sweden. The infrastructure in France and Spain was too weak to support a large German ground army. And the same could be said for Turkey. The Allies don't even need to stay and fight to occupy these countries. They can send their forces in, destroy the mines and mining equipment with enough TNT to make it impossible to excavate them for years, and then leave. And German industry would have ground to a halt, and the war would have ended a year earlier. Millions of lives would be spared compared with the OTL.