Admittedly, I don’t have much experience playing Europe only. I would imagine that the US goes to war in most cases at the collect income phase of US3 (no early entry). Being that the US is pretty much tied down at port in Eastern US until at war, that means that US4 it normally moves fleet to Gib sz91, then threatens a landing US5, and axis need to prepare multiple def.
The thing is that the US could land in Spain US4 (w/min loses to the Spanish defenders) instead of preparing to invade the following turn. This would be a full turn sooner, possibly catching the axis off guard. Because of the turn order, on UK4 the English can also put some boots in Spain (from Canada if necessary using Can NB to sz91), and land the entire RAF to defend the new American colony. I can’t see the allies loosing Spain any time soon. The axis aren’t going to be loaded up in S France, they typically load up in Paris, with inf/art, and W Germany with the same, plus some mech for mobility, but they are normally expecting a US turn 5 landing on the beaches of France, Norway, Italy, or N Africa so may not have bulked up yet (last push to Russia, before setting up defenses in W Europe). Fast moving units in W Germany can only get to the French coastal territories, but not to Spain itself, so you should be able to hold Spain to build a US minor IC in Spain, and an Air Base there would be cool to protect your transport fleet (depending on your needs). sz91 would still be the optimal sz to invade Spain because the UK naval base in Gib services sz91 (shuck-shuck), and an English air base built in Gib would help defend the combo fleet in sz91 until a Spanish air base could be built (depends on how the UK is doing in the Med, and how safe Gib is).
You should be able to bring in more reinforcements (US/UK troops, and more air) the turn after you take Spain as well, and can decide if you want to build an IC or air base in Spain. Should you advance into the French coastal territories to grab those coastal IC’s (could be difficult to hold), or set-up buffer/kill zones there allowing you to bulk up in Spain. The US will need to keep the sea lanes open having transports coming and going every turn for reinforcements, and an air base in Spain would help protect them. You have the UK naval base at Gib sz91, so you don’t necessarily need to build a US naval base in Spain for the shuck-shuck. It also forces the Germans into a 2 front war a turn earlier, so it should help relieve pressure on Russia.
Depending on how the UK is doing in the Med, they might be able to kill off some of the Turkish units before the axis claim them, but the damn Swedes are going to throw in with the Germans LOL (just for you Razor). My concern with this strat isn’t can I hold Spain, but that opening up Turkey could aid the axis getting into southern Russia, and the oil territories in the Mid East. Italy could be a real problem if the UK doesn’t knock them down before the neutral invasion. The UK/Russia could have a hard time holding this theater, but then Germany could be fighting a 3 front war so IDK.
The US can use some of their mech in Central US to take Columbia (undefended) the turn they attack Spain, then claim Brazil the next turn. Add a little air power and the US will conquer all of S America w/o using transports lol. Then transport any survives to Europe later.
I know that the Axis can foil this plan if they know its coming by building mech in Paris, or stacking S France etc…. but they are pretty vulnerable on the French coast if you don’t go for Spain. Plus the first time it is done, you should catch them off guard (like anything else), so they will be reacting to you, instead of setting the tone for your probable landing spot. It might be a one trick pony, but after that the possibility could stretch the axis defenses even further in future games. It could also force the axis to build up more defenses in W Europe, taking away from Barbarossa a turn earlier then they would like…
Plus the Western Allies could defend Spain drawing in axis forces, and can still hit any of the coastal targets that they normally could from sz91, so the axis still need to def multiple territories.