As stated in the title, what makes some cultures more resilient to waves of foreign conquerors than others. For example: Persian assimilation of Turks and Arabs, Greek assimilation of Romans and Italians, Chinese assimilation of Central Asian nomads, and Roman assimilation of Germanic tribes. While the above examples occurred due to different specific reasons, are there some common underlying themes that correlate with resilience? For example, why did Persians eventually influence the Islamic Civilization and Golden Age to such a high degree, and not other cultures in the Islamic Sphere such as Indians, Egyptians and even Assyrians (arguably also from equally ancient cultures)?
Not sure if this will count as a complete answer of your question but I'm currently studying the Holocaust and according to Daniel Goldhagen, the purpose of the emancipation of the Jews in Germany during the mid to late 19th century was for them to assimilate into German society and culture to a degree where the "Jews" would just disappear.
Quote from the book Hitler's Willing Executioners: Ordinary Germans and the Holocaust as follows:
The "liberals" the "friends" of Jews, shared the central tenets of the antisemites' image of Jewishness. Even while arguing for Jewish emancipation and then for full equality of Jews, they too believed and explicitly argued that Jews were different from, opposed to, and deleterious to Germans, that the Jews were alien to Germany, and essentially, they should disappear.
He further goes on to state that:
Liberals continued to defend Jews in the first half of the 19th century on the ominous basis of affirming the Jews' capacity for moral and social regeneration.
So in short, at least in Germany before the Holocaust, there was a widespread belief that the "Jews" needed to be eliminated from society. This elimination, as Goldhagen notes, was supposed to take place by assimilating "Jews" into German culture to the point where the culture of the "Jews" that they saw as incompatible with German society disappeared.
I hope this helps!
Source: Daniel Jonah Goldhagen, Hitler's Willing Executioners: Ordinary Germans and the Holocaust pp.57-58