You need either something like Cyprus, which is relatively separate geographically or some sort of mass conversion to make this happen. Jews is Europe were a small minority overall, and while in certain places, like Salonika, they might represent the largest single group, they were still a minority of the total population. You could have a Jewish city-state somewhere, but it won't last - you need a hinterland to grow food at a minimum and preferably some other resources. Certainly you could have Stalin, after WWII depopulate East Prussia and then relocate the Jews in the USSR and areas controlled by the USSR to this new entity. Given the history of Jews in Europe, I have a hard time seeing them being able to sustain a state strong enough to resist the Christian states of Europe based on the OTL Jewish population even if almost all were in one place - hence a mass conversion to make the Jewish entity populous enough and large enough to survive the hostility of all the neighbors as well as deal with the Mongols from the east (depending on where the state is) or the Ottomans from the south.