I remember a few years ago reading a compilation about the holocaust, including many articles about Agunas and such Halachic questions that were caused by the holocaust. It included a Hashkafa section where it quoted R' Desler posing a reason for the holocaust, and although it was quoted to be in Michtav M'Eliyahu V.3, I have never been able to locate it. Does anyone know of a source where he discusses the topic? Thanks!
Perhaps you mean vol IV, pg 124:
We are struck dumb at the terrible destruction that has been visited upon us in our generation, and we ask ourselves: For what reason has God done this to us… What is this great wrath? Let us examine this.
The entire period that preceded the destruction was a time when the burden of the exile was lightened upon the shoulders of the Jewish people…
Clearly, the period of the Emancipation was ordained by God to serve as a preparation for us for the coming of the Messiah, and it was for this purpose that the burden of exile was lightened upon us… Since we have turned the purpose upside down, and instead of understanding the hint from Above to prepare ourselves for redemption out of joy and expanding our consciousness, we used the new situation to assimilate. Therefore there awaited us the well-known danger of preparation for holiness that is not realized – as explained above. (And the fact that the destruction came only now, even though the process of assimilation was one that had developed gradually for a long time, is because God is long-suffering and does not bring punishment until the measure of sin is full, and there is no longer any hope of [a positive] influence bringing about a repair.
He seems to be saying it was because we had gone irrevocably down the road of assimilation. Hashem gave us the Emancipation for the sake of our having more tools to bring His redemption to the world, and instead we took it the opposite direction. And that had Hashem continued waiting for us to reverse course, we would have been irrevocably lost.
These words could be taken as an attack on the non-Orthodox, as David Landau did in "Piety & Power: The World of Jewish Fundamentalism." However, given historical context, it is more likely he is talking about assimilation's inroads in the Orthodox community; that even in the yeshivos boys were leaving left and right to secular nationalism (non-religious Zionism) and Communism.
There is even a hint of condemnation of chareidi life for becoming too insular and not taking advantage of the opportunity G-d gave us. Calling the Emancipation "a preparation for us for the coming of the Messiah" sounds more like Rav SR Hirsch or Neo-Orthodoxy in general than the stereotypical Lithuanian Jewish positions. (But then again, his own roots in Kelm were not stereotypical Lithuania; Kelm ran a gymnasia for a while.)