This is an example of subject-auxiliary inversion.
We use the past perfect in the if-clauses of conditionals when we want to talk about something hypothetical in the past:
- If you had been at the conference, you wouldn't have enjoyed it.
The word if here is important, because it shows us that this is a subordinate conditional clause. However, there is another way that we can show this. We can leave out the word if, and change the position of the Subject and the auxiliary verb, like you would for a question:
- Had you been at the conference, you wouldn't have enjoyed it.
In the sentence above, the Subject you, and the auxiliary verb had have changed places. This inversion is enough to show that this is a conditional subordinate clause, so we don't use the word if here.
The Original Poster's example
The important section of the sentence is:
this personal experience would not have led me to attempt to write this book had there not occurred at the end of World War II an event unique in history.
In the Original Poster's example, the subordinate clause is at the end and the main clause is at the beginning. Let's swap them round so that they're easier to understand:
Had there not occurred at the end of World War II an event unique in history, this personal experience would not have led me to attempt to write this book.
The protasis (the subordinate clause), is the section in bold letters. It is an unusual sentence. It uses the same pronoun, there, which we find in existential sentences like:
The writer has used a presentational construction. In presentational constructions we use the dummy pronoun there as the Subject and move the "semantic subject" to the end of the clause. This makes the sentence more dramatic. The listener has to wait right till the very end of the sentence to find out what the semantic subject is.
So in the Original Poster's example, the grammatical Subject of the sentence is the word there. The so-called semantic subject, the thing that we understand that "occurred", is an event unique in history.
The writer could have used the word if to make this sentence:
- If there had not occurred, right at the end of world war II, an event unique in history ...
Instead they used Subject-auxiliary inversion. Notice that - as well as dropping the word if - the Subject, there, and the auxiliary, had, have also changed places:
- Had there not occurred, right at the end of world war II, an event unique in history ...
This kind of Subject-auxiliary inversion is also quite common in conditionals which use the modal verb should:
- Should you see Bob, can you ask him to come and see me.
We also find it sometimes with other modal verbs, but this is very rare indeed:
- Could he but only change the past, he would.