A wartime papacy would be fascinating. I don't see it in Ireland: even though self-governing at the time, I suspect Orangemen in the Six Counties and in Scotland would have made enough noise to make that uncomfortable. New York...OK, the US was more Catholic-tolerant by WW II, but it's difficult to imagine the pope setting up a temporary Holy See there. I could, however, imagine a more heavily Catholic city like Boston, Philadelphia, or Baltimore. As to anywhere south of the Rio Grande, perhaps Buenos Aires or Rio: especially Buenos Aires, since that was pre-Peron Argentina, with a standard of living / development essentially that of contemporary Canada.