The differences between the Sephardim and Ashkenazim are traced to the different social, religious, economic, political, geographic, and even ethnic influences existent in each community. The differences are too numerous to enumerate in detail. Differences in practice in Ashkenazi and Sephardi schools of thought are also often argued to reflect the differences in Palestinian and Babylonian practice, which influenced the two former groups.
Assimilation and Daily Life
The Christian Reconquista altered the political and religious environment of Jews in Spain. While this eventually caused Jewish life to reflect the type of restrictions common in Franco-German regions, Sephardim were certainly more influenced by Islam than by Christianity.
Jews under Islamic rule in Spain underwent varying degrees of Arabization or even Islamization which to a large extent reveals the ability of non-Muslims to integrate to a large measure in Islamic society.
The Ashkenazim as the only notable minority in Christian lands experienced a very different environment. That should not by default create an image of constant isolation or persecution, but the relationship with Christianity society was certainly more restricted, though exceptions did exist. The everyday life of Jews in 11th century France for example points to a more complicated relationship with their Christian neighbors than typically imagined. Esra Shereshevsky states:
“The Jews of Troyes lived in harmony with their Christian neighbors; they hired Christian laborers under contract, had their horses shod in workshops owned by Christians and their clothes laundered and repaired by Christian tailors. Jews borrowed money from Christians and the Christians in turn supplied the Jews with fresh fodder for their cattle on Jewish holidays when Jews were not permitted to perform the chores entailed in fodder getting.”
Rabbi Yehudah HeChasid’s Sefer Chasidim also reveals a complex relationship with non-Jews. In his ethical work, Rabbi Yehudah includes a number of positive comments regarding friendships with gentiles, including praying for their safe journey, providing them with helpful advice, and even cautioning them when about to sin.
The Differences in World View: Economic
While the idea of Convivencia is perhaps exaggerated with the regards to the idyllic picture of Jewish life in Spain, the complex interrelationships of Muslims and Christians and Jews provided the latter with a much more open society. The differences between the Sephardim and Ashkenazim were however often as attributable to simple issues like economic reality to explain why variations in practice existed as they were to differences in religious philosophy and application. Because the differences are simply too many to full enumerate, we will review a few examples.
Day to Day Examples: Wine
The leniency of the Ashkenazim with regards to wine of non-Jews or Jewish wine handled by non-Jews is a perfect example. In France and Germany, many Jews worked in wine trade and viticulture. According to the Talmud wine handled by or made by non-Jews is prohibited as a beverage, nor may any benefit be derived from it. The prohibition extended rabbinically to even wine which was not definitely known to have been dedicated to an idol. The Biblical prohibition only restricts wine explicitly known to have been dedicated to an idol. The Shulchan Aruch composed by Joseph Caro retained the Talmudic prohibition. Moses Isserles, however, in his gloss to the Shulchan Aruch specifically mentions economic concern as the motivation for a more lenient application of the law.
“Since libation is not usual nowadays, some say that our wine touched by a non-Jew is prohibited only as far as drinking it concerned, but on may have a benefit from it. The same applies to setham yenam [the case of wine not definitively known to have been dedicated to an idol]. Because of this it is allowed to take their wine (setham yenam) as repayment of a debt since it is regarded as ‘rescue’ from their hands. Similarly, whenever a loss (of money) is imminent, e.g. if one has already bought (be-diabad) suspect wine, only may have a benefit from it. But one should not intentionally deal in the in the first instance (le-chathhilla) Some even permit this as well, but it is better to be scrupulous.”
In contrast, Maimonides applied the Talmudic restrictions completey when dealing with Christians. He did however, concede some leniency with respect to Arabs, though this appears to be in line with the view of earlier Geonim. The general prohibition against alcohol by Islam most likely provided the basis of the leniency.
Day to Day Examples: Terefah
Sephardi leniency in the area of terefah is generally assumed. Geographical and economic influences on the observance of Terefah however reflected the different economic realities of the Sephardim. In some areas Ashkenazim owned slaughter houses, while in others they used the abattoirs of their non-Jewish neighbors. In the latter cases, non-Jews provided the cattle. In such cases, the ability to more thoroughly inspect the lungs of slaughtered animals gave these Ashkenazim the ability to be more meticulous. Animals which did not meet these requirements were simply provided to non-Jewish customers and hence no economic problems existed between Jewish and non- Jewish slaughterers. In the case of Castile for example, where non-Jews refused to eat meat slaughtered by Jews, the concern for financial loss caused Sephardic rabbis to rule in accordance with Talmudic law only, and not on stricter laws introduced by recent authorities.
Day to Day Examples: Hametz
Another example is that of selling hametz before Passover. Since many Jews in Poland and Germany traded in spirits or kept arendas (inns), the wholesale divesture of hametz could lead to financial ruin. The idea of selling hametz apparently developed over time. The first stage involved the actual sale of hametz to a non-Jew. While the amount received would likely been lower that its actual value, its sale at least curtailed a full loss. The second stage saw the sale of hametz to a non-Jew with the expectation that it could be purchased back after Passover. The third stage saw a sale of the hametz without its removal from the original owner’s premises or full payment, which remains in practice today. 
The Differences in World View: The Social Arena
The impact of Arabic culture on the Sephardim has already been noted. The impact on religious life can be seen in the example of lustrations. While the practice of lustration had largely been abandoned by Talmudic times, it remained alive in certain regions including Babylonia and in Spain. The most likely reason for this continued observance is in the fact that Arabs performed lustrations meticulously. Since this act was considered a sanctification of the Divine Name, many Jews were strict in their emulation. 
Perhaps the clearest difference in how Christian and Muslim society impacted Jewish communities is in the area of marriage and sexuality. The takkanah of Rabbi Gershom prohibiting polygamy was not generally in effect in Spain nor in the Sephardic Diaspora communities following the expulsion. In fact in Turkey, some Ashkenazim yielded to Sephardic influence and adopted the practice of marrying second wives. Islamic norm and the Bible’s own approbation of such practices allowed them to continue, though curtailing polygamy was attempted through the insertion of a non-polygamy clause in Ketuboth or through an oath taken by the groom. In the case of Rabbi Gershom, Christianity clearly rejected this practice, kingship perhaps being the exception, and in Ashkenazi circles little if no responsa on the subject was produced. Sephardim in Spain also retained concubinage and out right prostitution, a practice unheard of in Ashkenazi circles and perhaps in part why the latter even received praise from Christians for their moral virtue.
The Differences in World View: Martyrdom
Perhaps one of the best examples of how outside religious and social influences impacted both Ashkenazi and Sephardic communities is in the response to persecution. In Christian Europe, martyrdom or suicide was largely the preferred response to forced conversions. Whether this attitude toward martyrdom was as Mark Cohen asserts an elaboration of the martyr traditions recorded in the Midrash is unclear. Jacob Katz argues that Ashkenazic views of martyrdom were also influenced by their conviction that Christianity was idolatrous. Their views were further influenced by the willingness of Christians to suffer martyrdom. If Christians were willing to suffer martyrdom, so should Jews. Cohen also notes that conversion to escape death likely occurred to a greater degree than alluded to in Hebrew accounts of the period, but that martyrdom remained the Ashkenazic ideal. Nevertheless, Rabbi Yehudah HeChasid’s references to both apostates and various acts of dissimulation in his Sefer Chasidim make it clear that the situation was more complicated than is often appreciated.  Rabbi Yehudah for examples points to an example in Rokeach 316 and in Teshuvot Maharil 118.
“When the members of his community were offered the alternative of either converting or being killed, he [the rabbi] advised them to convert and afterwards to return to Judaism…when things settled down, they all returned to Judaism. Nevertheless, since the rabbi counseled his flock to defect from the Jewish faith, his offspring all became apostates, and he is being punished [in the hereafter] as though he was the one who had caused them to sin.”
The Sephardim and Persecution
Jews living under Islamic rule had very different responses. According to Menahem Ben-Sasson, Jews underwent extensive cultural Arabization. As a consequence, Jews did not maintain the same invectives against the dominant religion that Ashkenazic Jews maintained toward Christianity. More importantly, the concept of martyrdom was not a similarly significant concept in Islam as it was in Christianity. As Cohen notes, in Islam, the martyr is a warrior who dies fighting in a holy war. Cohen states:
“Confronted by religious persecution, Muslims favored outward accommodation or dissimulation, in Arabic taqiyya while inwardly maintaining belief in Islam. Rather than having a gentile model of martyrdom to emulate and even surpass, the Jews of Islam seem to have been influenced by the Islamic response to forced conversion in their own pattern of accommodation.”
In addition, in all three great Islamic persecutions of the medieval period (i.e. the persecution under al-Hakim, the Almohads, and the persecution in Yemen” Jews and Christians who were forcibly converted were eventually allowed to revert to their original faiths. The influence of taqiyya is I believe evidenced in the writings of Maimonides and his father in response to forced conversions of Jews of Morocco and Yemen. Abdul Hamid Siddiqui refers to the words of Ibn Abbas, a Sunni commentator:
“al-Taqiyya is with the tongue only; he who has been coerced into saying that which angers Allah (SWT), and his heart is comfortable (i.e., his true faith has not been shaken.), then (saying that which he has been coerced to say) will not harm him (at all); (because) al-Taqiyya is with the tongue only, (not the heart).”
It is clear then that for Islam, the survival of a faithful Muslim is of utmost importance.  The ability to feign loyalty to another religion is not considered problematic when faced with persecution. Responding to the situation of Moroccan Jews, Maimonides states the following:
“Now if he did not surrender himself to death but transgressed under duress and did not die, he did not act properly and under compulsion he profaned G-d’s name. However, he is not to be punished by any of the seven means of retribution. Not a single instance is found in the Torah in which a forced individual is sentenced to any of the punishments, whether the transgression was light or grave…”
In this particular case as well as in Maimonides’ letter to the community of Yemen who underwent a similar persecution, the fact that Islam was generally not considered as idolatrous was certainly beneficial to this lenient view. In addition, “conversion” to Islam only required recitation of the shahada or testimony and not the more involved procedures of Christian initiation. This certainly parallels the Islamic examples provided by Siddiqui. Admittedly Islamic practice was also much closer to Judaism than Christianity was which allowed for a greater degree of obscurity when practicing Jewish customs in the case of Maimonides’ letters to affected communities. Cohen states:
“Jews could accept Islam outwardly, demonstrating their conversion by attending Friday prayer and avoiding acts disapproved of in Islam, while secretly adhering to Judaism in the privacy of their homes.”
In addition, at least one Islamic source of the 10th century upholds the view that the forced conversion is not binding unless the convert has overcome their fear.  Taqiyya is a fundamental Islamic concept based on the Quran. The Quran holds blameless Muslims who disguise their beliefs in cases of safety.
To what extent did the notion of Taqiyya influence Jewish communities in the period seeing the transition from Muslim to Christian authority is unclear, yet deserves a closer review. In the case of Christianity, the dominant Jewish perspective was certainly much more aligned toward the view that it was in fact idolatrous which makes the Maimonidean view toward forced conversion more problematic. Nevertheless, the extent of cultural influence is not something which devolves instantaneously.
The differences between Ashkenazim and Sephardim of the medieval period extend to a whole host of other areas including their liturgical practices, approaches to the Biblical and Talmudic study, manner of dress, and of course differences in Hebrew pronunciation. The differences often resulted in identity struggles which manifested themselves in conflict and animosity towards each other. These conflicts remain to some extent in and outside the Land of Israel.
The differences were in large part reflective of very different cultural surroundings and the impact of the dominant political and religious cultures of Christianity and Islam. They were however also based on the older influences such as the differences between “Palestinian” and “Babylonian” practice, though neither group was completely reflective of either tradition. The influence and movement between rabbis of note and rabbinic literature ultimately impacted both societies over time and the cross pollination between both communities is extensive.
If you are intereste in learning more, one of the best books on this topic is H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa,
 H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa, (London: Oxford University Press, 1958), 233. The presence of Jews in the Iberian Peninsula is attested to as early as the fourth century of the Common Era as evidenced by anti-Jewish legislation enacted under the Council of Elvira. Some of the edicts of the Council prohibit intermarriage which apparently occurred in sufficient numbers to warrant their attention. Joseph Perez, History of a Tragedy: The Expulsion of the Jews from Spain, (Chicago: University of Illinois Press, 1993), 5.
 H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa, (London: Oxford University Press, 1958), 210.
 According to Menahem Ben-Sasson, Jews underwent extensive cultural Arabization. Ben-Sasson, “On the Jewish Identity of Forced Converts: A Study of Forced Conversion in the Almohade Period.” 20. Bernard Lewis emphasizes the linguistic transformation in Jewish life with the rise of Islamic rule:” One of the major changes that took place Jewish life in these countries was the process of Arabization, meaning primarily but not exclusively the replacement of the older languages by Arabic.” He also refers to a number of similarities between the two religious traditions: the similarity of a rabbi to an alim, the similarity in emphasis of religious law (halakhah and sharia), in teshuvot and in the Islamic fatwa. The influence on Jewish poetry, philosophy and theology is apparent as both arose in Islamic lands. Bernard Lewis, The Jews of Islam, (Princeton: Princeton University Press, 1984), 76, 79-80.
 Esra Shereshevsky, Rashi: The Man and His World, (Northvale: Jason Aronson, 1996), 60.
 Avraham Yaakov Finkel, trans., Rabbi Yehudah HeChasid: Sefer Chasidim, (Northvale: Jason Aronson, 1997), 365, 375-376.
 H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa, (London: Oxford University Press, 1958), 210.
 Avodah Zarah 29b.
 Yoreh Deah., cap. 123, no. 1. H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa, (London: Oxford University Press, 1958), 210.
 Ibid., 211.
 H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa, (London: Oxford University Press, 1958), 200.
 Ibid., 215.
 Ibid., 215.
 Ibid., 215.
 Ibid., 215.-216.
 H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa, (London: Oxford University Press, 1958), 228.
 Ibid., 167-168. There were however communities in Aragon in the 13th century that adopted this prohibition.
 Ibid., 63.
 Ibid., 253.
 Ibid., 253-254.
 Mark R. Cohen, Under Crescent and Cross: The Jews in the Middle Ages, (Princeton: Princeton University Press, 1994), 174.
 Here we do find some contradiction as the application of the laws of Avodah Zarah in the 12th and 13th centuries were applied more leniently at least in the area of trade with non-Jews. The principal essentially laid down the view that the Gentiles of the region did not worship idols. H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa, (London: Oxford University Press, 1958), 209.
 Mark R. Cohen, Under Crescent and Cross: The Jews in the Middle Ages, (Princeton: Princeton University Press, 1994), 175.
 Avraham Yaakov Finkel, trans., Rabbi Yehudah HeChasid: Sefer Chasidim, ( Northvale: Jason Aronson, 1997), 349-359.
 Ibid. 349.
 Ben-Sasson, “On the Jewish Identity of Forced Converts: A Study of Forced Conversion in the Almohade Period.” 20.
 Mark R. Cohen, Under Crescent and Cross: The Jews in the Middle Ages, (Princeton: Princeton University Press, 1994), 176. Abdul Hamid Siddiqui defines Taqiyya as “Concealing or disguising one’s beliefs, convictions, ideas, feelings, opinions, and/or strategies at a time of eminent danger, whether now or later in time, to save oneself from physical and/or mental injury.” http://www.al-islam.org/encyclopedia/chapter6b/1.html. See also Vincent Barletta, Deixis, Taqiyya, and Textual Mediation in Crypto-Muslim Aragon, Text and Talk, 2008.
 Ibid. Chapter 6b Siddiqui also refers Abd al-Razak, who in his book “al- Dala-il,” wrote: “The nonbelievers arrested `Ammar Ibn Yasir and (tortured him until) he uttered foul words about the Prophet, and praised their gods (idols); and when they released him, he went straight to the Prophet. The Prophet said: “Is there something on your mind?” `Ammar Ibn Yasir said: “Bad (news)! They would not release me until I defamed you and praised their gods!” The Prophet said: “How do you find your heart to be?” `Ammar answered: “Comfortable with faith.” So the Prophet said: “Then if they come back for you, then do the same thing all over again.” Allah at that moment revealed the verse: “….except under compulsion, his heart remaining firm in faith…[16:106]”Siddiqui also refers to what is narrated in al-Sirah al-Halabiyyah, v3, p61, that: “After the conquest of the city of Khaybar by the Muslims, the Prophet was approached by Hajaj Ibn `Aalat and told: “O Prophet of Allah: I have in Mecca some excess wealth and some relatives, and I would like to have them back; am I excused if I bad-mouth you (to escape persecution)?” The Prophet excused him and said: “Say whatever you have to say.” In addition, alal al-Din al-Suyuti in his book, “al-Durr al-Manthoor Fi al-Tafsir al-Ma’athoor,” v2, p176, narrates that: Abd Ibn Hameed, on the authority of al-Hassan, said: “al-Taqiyya is permissible until the Day of judgment.”
 David Hartman, Crisis and Leadership: Epistles of Maimonides, (Philadelphia: Jewish Publication Society, 1985), 29.
 Mark R. Cohen, Under Crescent and Cross: The Jews in the Middle Ages, (Princeton: Princeton University Press, 1994), 176.
 Ibid. 176.
 Qur’an 16:106
 H.J. Zimmels, Ashkenazim and Sephardim: The Relations, Differences, and Problems as Reflected in the Rabbinical Responsa, (London: Oxford University Press, 1958), 61.
Posted by Rabbi Dr. Juan Marcos Bejarano Gutierrez the director of the B’nei Anusim Center for Education. He is the author of The Converso Dilemma: Halakhic Responsa and the Status of Forced Converts