Something may be a one-time event, but that doesn't mean it's capitalized. I would refer to "the assault" throughout unless you're using the book's title. "The assault that takes place in The Assault assulted my senses."
In English, aside from capitalizing the first word of a sentence, the only time words are generally capitalized if they are proper nouns or titles:
- Names ("John", "New Jersey", etc.)
- Formal titles when used as a form of address ("Doctor, it hurts when I do this" and "Mother, will you pass the salt?" but "My doctor says to never, ever stand on my head" and "My mother passed the salt.")
- Titles of books, movies, and other works ("Arguing that the empire portrayed in The Empire Strikes Back is representative of the Roman Empire is a weak argument.")
There are more arcane rules of capitalization for edge cases, and the style book you're using can affect how you handle some situations. (For example, do you treat the text after a colon as a new sentence or a fragment?) Here are a couple of good examples that are freely available:
Of course, there are informal situations where you can decide to capitalize a word regardless of these rules. Fiction will sometimes do this, and I do it in emails for effect to point out The Big Concepts, but this can be overused far too easily.
Above all, pick a method and stick to it.