a challenging challenge indeed---
First, we need to define what we mean by Sephardic culture, either the Iberian Jewish tradition, or if we are using it as a broader term that groups them with all the various Jewish subcultures of the Muslim world. Strictly speaking the designation for North African and West Asia Jews is Mizrahi.
Either way, the predominance of the Ashkenazi tradition is inherently part of larger demographic trends. It would be difficult to stop Ashkenazi predominance, and nearly impossible to end up with Sephardic and or Mizrahi dominance, without changing the demographic trajectory of their surrounding gentile populations in a massive way.
The rise of the Ashkenazim was part and parcel of the explosive demographic growth of northern and eastern Europe over the last 2000 years. The relative decline of the Sephardim and Mizrahim was related to, and likely driven by factors that caused the relative demographic stagnation of the Middle East and North Africa.
A comparison of differential population growth between fairly representative set of countries that were home to the Ashkenazi tradition, Germany and Ukraine on the one hand, and another set of countries home to a significant share of the Sephardic and Mizrahi population, Turkey, Egypt, Iran and Spain, on the other.
In the first year AD, the combined population of Germany and Ukraine is estimated to have been about 7 million. The combined population of Iran, Turkey, Egypt and Spain was nearly triple at 19 million.
See linked .pdf map illustrating relative populations and listing the top 20 countries by population in 1 AD. (
http://www.worldmapper.org/posters/worldmapper_map7_ver5.pdf )
By 1500 AD, the combined population of Germany and Ukraine had reached 19 million, nearly tripling over the intervening time period. Meanwhile, the combined population of Iran, Turkey, Egypt and Spain was still higher at 21 million, but the net increase was a relatively paltry 2 million over the intervening millennium and a half.
See linked .pdf map illustrating relative populations and listing the top 20 countries by population in 1500 AD. (
http://www.worldmapper.org/posters/worldmapper_map8_ver5.pdf )
By 1900 AD, the combined population of Germany and Ukraine shot past the Mediterranean and West Asian worlds, more than quadrupling to 82 million. Meanwhile, the combined population of Iran, Turkey, Egypt and Spain increased only 2.5 times, to 54 million.
see -
http://www.worldmapper.org/posters/worldmapper_map9_ver5.pdf
Over the two millennia, Germany and Ukraine had massively increased agricultural productivity, supporting a population surge, far more than the more than the southerly nations that hosted the bulk of the world’s Jewish population in classical and biblical times.
The Jewish populations of these regions correlated with the general populations.
Individual national scenarios accentuated this even more. With Spain’s expulsion of Jews in 1492, and the follow-on inquisition resulting in the further suppression or exile of converses and crypto-Jews, and the exclusion of Jews from Portugal as well as the Iberian colonies, none of the demographic growth within the Iberian world increased the Sephardic population.
Instead, Sephardic Jews ended up for the most part reinforcing the total Jewish population of the Muslim world, in many individual countries outnumbering longer established Mizrahi communities. Other Sephardim moved to the Netherlands, Britain, and their American colonies and to a small extent France, but they were first numerically, and then culturally, overshadowed by Central and Eastern European Ashkenazim.
Note also, that Egypt and Iran had fallen out of the list of top 20 most populated countries by 1900, while Poland, home to a huge Jewish (and even larger Catholic) population joined the top 20 by 1900.
So, how to answer the challenge –
The most direct and simplest method is also the least plausible. Prevent East-Central Europe from becoming a relative demographic juggernaut compared to the Middle East.
It would be interesting, and perhaps also gruesome, to examine in-depth the mechanism that saw the Pale of Settlement explode demographically while the Middle East stagnated. Both regions were backward relative to Western Europe after 1500. The Pale of Settlement was not any more urban than the Middle East, and its households at any class level did not have a reputation for being particularly more prosperous. However, the environment had something which allowed populations in the former to grow while the latter barely did.
It would seem to me that relative abundance of water in the Pale, and everything that led to, more arable land and greater food production, is what gave it the edge over more arid regions.
I am guessing that despite the poverty of individual households in the Pale, there were just always more calories around to consume, possibly allowing for earlier puberty and most of all more live births. The amount of infant and childhood deaths were huge, but the environment gave population growth there a large head start, and perhaps it influenced lifestyles, for instance,
more of the population getting married at younger ages, fewer peop
le being stuck in celibate lifestyles, less use of herbal/folk medicine birth control methods or induced abortions, etc.
So, if we could get massive soil erosion and enduring desertification of the Pale of Settlement, a byproduct would be the drastic limitation of Ashkenazi growth and, in turn, influence.
But that scenario is not only too hard for me to figure out how to achieve environmentally, it also results in changes far broader reaching, and interesting to most readers than the relative size of different Jewish subcommunities. It torpedoes the history of the Polish Commonwealth and Russian Empire completely.
Conceivably, the Polish and Lithuanian and Hungarian Kings might have gone a different way fallen in with the expulsion mania of western Europe and some German states and Muscovy. However, to work this out, we would need to change the incentives that made rulers in east-central Europe choose to be more tolerant of Jews than the westerners during the timeframe from the Crusades to the 1700s. I’d be interested in any ideas people would have. A substitute merchant class of some sort?
One of the methods that just might make Sephardic (though not Mizrahi) culture more predominant over Ashkenazi would be if the Reconquista, Expulsion and Inquisition in Spain were all accelerated by one to four centuries. In the period between 1096 and 1453, the Muslim world was less stable and probably less attractive to Sephardic emigrants than the OTL Ottoman Empire of the 1500s was. Meanwhile, Poland, Hungary and Lithuania were growing mining and agricultural frontiers. Perhaps an earlier expulsion from Spain could have resulted in a much more substantial Sephardic flow to eastern Europe, perhaps “capturing” predominance of the emerging Jewish communities of the region from emigrants from Germany. If the Sephardim made up “the mostest” of Jewry of East-Central Europe, arrivals from northwest Europe might have been more likely to assimilate to a Sephardic derived culture than to assimilate Sephardim into the Ashkenazi culture (as likely happened to some extent in OTL).
Paradoxically, if, instead of expulsion, the Spanish Empire had been more like the latter Russian empire and permitted the continued existence of Jewish populations in at least some provinces in Iberia (Perhaps Castille is made judenfrei, but Portugal, Navarre and Aragon allow jewish communities to continue, under discriminatory laws to be sure). A Sephardic “Pale” would likely not have led to an overshadowing of Ashkenazism, but would have been far more in touch with European developments and Spanish developments, certainly than those who went to the Middle East.
Even if officially excluded from the American empire, Jews living in Iberia would have had a better chance of slipping through to the Americas. Spanish policy might conceivably have changed to permit migration during the eighteenth century. At a minimum, Sephardic jews in Iberia probably would have spread their culture into Latin America post-independence, and certainly taken part in the late nineteenth century wave of Iberian emigration to South America, so we would not have the irony of OTL that most Latin American Jews trace their heritage to Germany, Poland or Russia rather than Spain or Portugal.