Alma 55:1

Now it came to pass that when Moroni had received this epistle, he was more angry, because he knew that Ammoron had a perfect knowledge of his fraud; yea, he knew that Ammoron knew that it was not a just cause that had caused him to wage a war against the people of Nephi.

When Moroni got this letter, he was more angry. He knew that Ammoron understood that he was lying. Ammoron knew that it was not a fair reason to start a war against the Nephites.

When Moroni read Ammoron’s letter, his anger flared. He knew Ammoron understood perfectly well his own dishonesty—that his war against the Nephites had no just cause behind it.