Here's a historical What If? I was just thinking that the Allies took French Morocco and Algeria in Operation Torch in November 1942 and met up with Montgomery's army moving in from the East. They took Tunisia and then Sicily was obviously the next step. What if, though, they'd invaded Spain rather than Italy next, in early or mid 1943?
Consider: We know, with 20/20 hindsight, that the Italian campaign was, if not a complete disaster, certainly not a success. A lot of Allied troops got tied up in Italy, on territory that is not exactly propititious for an attacking army, and the Germans sure chewed them up. We can see on the map that Italy is very mountainous and easy to defend, so that should have given us a clue. Spain is mountainous, too, but the example of the Spanish Civil War should have given us a lot of examples on how to successfully campaign in Spain. One reason for invading Italy was to knock Mussolini out of the war, but by then we knew that Mussolini was a paper tiger. He could wait. Hitler was the enemy.
Now, I need to know things that I'm not going to bother to look up to actually make this plausible. I need to know:
what forces the Allies had available to invade Spain. Let's assume that you had eight infantry, two airborne, and three armored divisions to work with.
what forces Franco had to resist with. Let's say virtually nothing, since Spain was completely broke, still half-destroyed, and deeply divided politically, and had no navy, air force, or armor to speak of.
what forces the Germans had in southern France, how quickly they could get into Spain, and what other units the Axis could move into the peninsula. Let's say two armored divisions and ten infantry could make it across the Pyrenees on fairly short notice.
We don't know those things, but I think my assumptions are fairly realistic.
So here's what you do. You pulled off the North Africa landings with virtually no interference from the German U-boats and air force, but you have to figure on some losses going in to both German subs and the Luftwaffe. Still, you should be able to surprise them; I don't think they're going to be expecting this one too far in advance, and you know from breaking their codes that they're not ready for such a move.
You send in commando units and airborne brigades to seize the ports of La Coruna/El Ferrol, Gijon, and Santander in the north. If you have real stones and more units than I've calculated, grab Bilbao too. With a bunch of Royal Navy and US battleships and cruisers off the coast and with the air forces flying from Morocco and Britain, you blast Spain's air force off the map and hit the Luftwaffe over France. Use your heavy bombers to knock out transport centers in southern France in order to impede the Germans from moving quickly; then go back to bombing the Ruhr the next night. Now that you've got the ports, send in a couple of armored divisions and about six infantry--they can just land there, you don't have to worry about resistance--and drive on Bilbao and the pass into southern France at Irun. Send a division toward Madrid from the north. In order to wipe up the Spanish regime, also seize the ports of Algeciras and Cadiz in the south with an armored division and whatever infantry you've got left. Drive up toward Madrid by way of Extremadura and Toledo just like General Franco did in '36. The regime will fall in a couple of weeks, if not sooner. Expect a bunch of wacky Falangistas to try to put up a fight at, say, Puertollano, and get plowed over. Resistance should cease.
Now, what do you do if you're the Germans? Realistically, you have two choices: dig in at the Pyrenees or move south into Spain. Really the only obstacle in front of the Pyrenees is the Ebro River, which would also be a good defensive line. Meanwhile, more Allied units and tons of supply are pouring in at the ports of both northern and southern Spain. If you're the Germans, you do not try to fight them in the open on the Spanish plain because there are a lot of Allied units coming at Madrid from both north and south. Allied bombing is interrupting your access to the battlegrounds by blowing up bridges over the Garonne and railroad junctions all over the place. The war in the air over France would have been bitter and bloody and rougher in, say, July or August 1943 than what we saw in June 1944, since the Luftwaffe wouldn't have been as weak a year earlier, but I think the Allies would have won.
So, me being the Germans, I'd try to hold at the line of the Pyrenees. No point in trying to prop up the Franco regime with about fifteen Allied divisions rolling through and more on the way, coming in through the northern Spanish ports, and the Ebro line is longer and less defendable than the Pyrenees. There will probably be a very rough battle around Irun and Bayonne as the Allies pour through to the west of the Pyrenees, and maybe another one around Portbou and La Jonquera on the east side. I think the Allies would have won, though, and at August or September 1943 you'd be on the French side of the Pyrenees. From there you ought to be able to launch an invasion of Western Europe much more efficiently than from the capture of a defended bridgehead, like at Normandy. You wouldn't be throwing your guys away like in the fight through Italy, and you'd have the sum of both the troops that wound up in Italy and those that wound up in France striking together at the Germans, and probably a lot of useless Italian troops too.
If we'd done this we might have considerably shortened the war at less cost, and a lot of people who got killed might have survived.
What's our moral justification for attacking neutral Spain? Well, Spain wasn't neutral, it was officially "non-belligerent". Spanish "volunteers", the Division Azul, were fighting against the Soviets on the Leningrad front. Spain was openly pro-Axis diplomatically, and it had a dictatorial government that had been put in place, at least partially, by Hitler and Mussolini. Those are all casus belli (help, Dr. Weevil, what's the plural?) in my book.
So what do you think? I'm expecting a hail of brickbats, rotten tomatoes, and dead cats...