I agree that WWII was an antecedent causal factor. It set the stage. But the Church does not collapse because of external factors. When the faith is strong, then external circumstances just make it even stronger.
The Allied/Communist victory in World War II destroyed the traditional basis of an exclusionary Christian society in Europe and America.
I agree that the Allied/Communist victory swept away traditional political foundations in some European countries. But the Church was still standing strong. It was in the 1950s when the Vatican signed a model concordat with Spain making Catholicism the only officially recognized religion. The Church was the last bulwark against the destruction of Christian society. Until Vatican II and the documents on Religious Liberty, Ecumenism, and Nostra Aetate.
Evelyn Waugh provided an outstanding analysis. His trilogy of WWII "Sword of Honour" (also called Men at Arms) described the "unconditional surrender" of European Christian civilization. But he saw the Catholic Church as the only beacon of light preserving all that was best. All the more reason why he was crushed when Vatican II came along and the Church sold out to all the phony theories that had already taken hold of the political situation.
So no one would agree with you more than Evelyn Waugh when it comes to the role of WWII in the destruction of traditional Western Christian civiliation. But he was very clear that the Church had remained immune until Vatican II. The trilogy was written throughout the fifties, but when he went to write a new preface for a one-volume edition in 1964, he wrote:
"On reading the book I realized that I had done something quite outside my original intention. I had written an obituary of the Roman Catholic Church in England as it had existed for many centuries.... It never occurred to me, writing Sword of Honour, that the Church was susceptible of change. I was wrong and I have seen a superficial revolution in what then seemed permanent. "I can't find the rest of the preface on-line, but this quote is from an article by Ralph McInerny who goes on to comment:
"Waugh was a convert to Catholicism, and like many for whom the doctrine, ritual, and culture of Catholicism represented the very antithesis to modern decadence, he found Vatican II a severe test, if not of his faith, at least of his loyalty. He was appalled by the sanctioned and unsanctioned changes made in the wake of the council. He communicated his views to John Cardinal Heenan, and their correspondence has been published... The fact that the very aspects of the modern world from which he was fleeing should suddenly be regarded as worthy models for the Church elicited memorable prose from Waugh."
Not so! Pius XII was full speed ahead for collaboration with the Allies/Communists despite the fact that Catholic Europe was on the other side (willingly or not). Fish rot from the head. As Mary Ball Martinez put it in "The Undermining of the Catholic Church" (I can't reccomend this book enough) of Pius XII in WWII:
"His priority? Jews, not Catholics."
Lest we forget, it was Pius XII who appointed Bugnini and Antonelli in 1947, and Pius XII who initiated the planning of Vatican II, carried into effect by his less than forthright sucessor pretending that he had come up with the idea on his own. Judaizing agents of B'nai Brith and the AJC like Cardinal Bea and Fr. Malachi Martin were appoitnees of Pius XII.
Yes, the Church, considered as a whole can and will always preserve all that is best. But the institution has been in revolutionary hands since the death of Pius XI, following a course plotted by Cardinal Rampolla at the turn of the century. Outwardly, this no longer can appear so.