Note: I originally composed this as a follow-up question to the thread "Why is D-Day always called the second front when the Italian campaign had been going on for a year at the time?": -> but it seemed to grow in scope as I typed so I posted it here as a top level question. Unfortunately, I apparently strayed too far into counterfactual discussion territory so was asked to revise and resubmit. Here we go, sorry for any residual reddiquettal errors! :-)
I like to think that I'm fairly familiar with the history of WW II but the D-Day nomenclature thread made me realize that I've never really come across any research suggesting that the Allies even considered the possibility of attacking Germany from Italy by passing neither to the left (France) nor the right (Austria) but by barreling right up the middle through Switzerland.
I do know that the Swiss have always been pretty conscious of their geographic location and military preparedness has always been high on the national ToDo list: -> but given the realities of geography and the state of the war by 1944, one wonders if there is any evidence to suggest that Allied planners had considered the possibility of trying to force the Swiss to choose sides and permit the transit of Allied armies into the Third Reich?
Considering the question more broadly, what does the historical record tell us about attempts by the Allies to pressure any of the various neutral non-combatant states (e.g. Switzerland, Sweden, Portugal, Ireland) to pick up a ball and get in the game?
For some (e.g. Sweden) neutrality was probably easier to justify than others (after all, Germany was closer to Sweden that any Allied state and after Norway fell the strategic situation called for prudence from the Swedes, to say the least) and Franco was obviously more sympathetic to the Axis than the Allied cause, but the bottom line is that the Allied track record in attracting partners from beyond the U.S., the British Commonwealth and refugees from conquered nations seems to have been poor, to say the least.
So what does history tell about what was tried and why were the Allies not more successful in bringing in non-combatant nations into the Allied fold, even towards the end when joining the winning team would seem to have offered some advantages in a post-war world?
I wrote this answer before your last question got deleted. But, I don't think you saw it.
That being said I can talk about what kind of pressure was put on the various neutral nations. Sweden and Switzerland both had long established traditions of neutrality. The only pressure on Sweden came from the allies pressuring Sweden to stop selling Germany essential Iron Ore which German industry relied on. The allies even considered seizing these mines from Sweden. Germany on the other hand forced Sweden to let Germany use its airpsace and use its railways to transport troops. German soldiers on leave could also go through Sweden from Norway. Besides those concessions no one expected Sweden or Switzerland to really participate in the war militarily. Ireland too was left alone besides the occasional accidental bombing raid from Germany. There was also a German plan to land in Ireland but it was never really a serious plan. The British too developed a plan where they would occupy Ireland if the Germans tried a landing.
Now, the major neutral nations that did actually get seriously courted were Turkey and Spain.
The Axis had never fought for Turkey's entry into the axis, but the allies tried to court them desperately. One of the motivations behind the Sicilian campaign was that it might show Turkey that the axis were losing and that they should join the allies. However, the Germans acted first and seized a number of islands in the Aegean sea which pushed Turkey further into neutrality. They did eventually end up declaring war a few days before Germany surrendered (so they could gain access to the UN). Spain was an authoritarian dictatorship and and Fransico Franco (the leader of Spain) owed much to Hitler and Mussolini who had given vital support in his Spanish civil war. Hitler and Franco met a small town on the Franco-Spanish border called Hendaye. Hitler assumed Franco would easily join the axis, but Franco was smarter than that. He purposely demanded concessions he knew Hitler couldn't make, for example Franco demanded French Morocco which Hitler couldn't afford to give him. The only thing Hitler managed to get was a volunteer division to fight the Soviets.
Sources:
Second World War by John Keegan
Third Reich at War Richard Evans
Churchill threatened Ireland with an invasion if they did not join and fight with Britain. Nothing came off these threats though when Ireland remained neutral