A pod that comes to mind is the Habsburg attempt at retaking the Hungarian Crown which fell through after Kaiser Karl's death in Portugal. What's the best the Habsburgs could do here assuming they manage to retake power in Hungary?
A restoration so early on is basically impossible. The new multi-national states have too much support and will do anything to prevent a Habsburg restoration as will Romania. Hungary is too weak, and will not have support of great powers. If Karl does retake power, Hungary will get a giant slapdown by a Czechoslovak-Romanian intervention. It will not even be a war, a pacification would be a more proper term. Karl either flees into exile again, or gets interned, or even shot.
How would the little Entente react here?
As late as 1935, the main focus of Czechoslovak intelligence was to concentrate on any Habsburg activity. From then division general Ingr: "I have no idea of German army formations, but I do know which Habsburg archduke has a mistress in Vienna, and which nobleman has had a dinner with Zita Bourbon." That fool Benes openly declared as late as Anschluss that anexation of Austria by Hitler is much better then a Habsburg restoration. The Yugoslavs and Romanians would interpret any restoration as a threat to their territorial integrity.
I think Dolfuss had plans to restore Kaiser Otto to the throne. I think that he had some plans with Mussolini for a limited Habsburg restoration. I think he wanted to have a marriage between Otto and a Savoyard princess with the rump Empire being a de-facto client state of Italy.
This was considered in 1938. Otto was in Austria and offered to take leadership and fight Germany. Hitler even had the ocupation of Austria named Fall Otto, and ordered his execution should Otto be caught. Many of the main Habsburg supporters (mainly aristocrats, the village population and the Austrian Jews), either immigrated or faced persecution (and worse for the Austrian jews of course). His Hohenberg cousins (the children of Franz Ferdinand), ended in Dachau.
The lack of support by Mussolini, who has decied to sacrifice Austria to curry favour with Hitler, with Otto fleeing the country and later organizing the Austrian exiles.
So, any Habsburg restoration would require either a) getting rid of the little Entente (not that hard), b) the little Entente being forced to accept Habsburg restoration (possible) c) little Entente going: "Actually, Habsburgs are fine, you can rule Austria and/or Hungary, if the people accept Otto!" (which would probably require God himself telling them to do it, and even then the Czechs probably will not)
So scenario a) is not that hard, it happened OTL. Let's say that the Germans are more sucessfull in USSR, with Soviets being more slow in retaking their territory. The WAllies manage to liberate the entirity of 1937 Germany, Austria, Czechoslovakia, Hungary, as well as modern day Bosnia,Croatia and Slovenia. Otto meanwhile managed to actually have the Austrian volunteer force be deployed in combat and participates in liberating Austria. Post-war, with much of pre-war Austrian politicians discredited, Otto is supported as head of state of Austria, and later on a referendum restores him to the throne. Meanwhile, with a pro-western Hungary, there is also good a chance of restoration there. Czechoslovakia will be unfriendly, but in no position to do anything. Croatia and Slovenia, considering that the rest of Yugoslavia is red, will most likely separate, and might also invite in Otto, though frankly this one is highly unlikely. So Austro-Hungarian personal union, perhaps including Croatia and Slovenia.
Scenario b) is basically the Mussolini plan. Mussolini supports restoring Otto in Austria (this would probably require pro-Allied Mussolini), though its a question if Otto
would accept. Perhaps an inclusion of Hungary and Croatia is possible, though highly unlikely as Mussolini will not want his puppet geting too powerfull, Savoyards will want one of their own in Croatia and Horthy would be terrified that Otto might decide that he is a traitor to the Imperial family.
The third option is quite unlikely, though possibly quite rewarding. Unfortunately I have no time to write on it now, as I my lessons are starting. Maybe later.
