This is the blockquote from the book that I read
They were about as smart as dogs, too. But what they lacked in brainpower, they made up for in sheer tenacity. He had seen them hurl themselves at their enemies until the ground was piled high with their corpses … and their opponents had depleted their ammunition.
I can't understand sequence of tenses in the last sentence. I see the grammar structure of sentence as follows:
[principal clause | past perfect] [subordinate object clause | present simple] until [subordinate adverbal clause of condition | past simple] ... and [subordinate adverbal clause of result | past perfect]
First clause (principal) is in past perfect tense because it describes events that happened before than other context (correct me if I'm wrong). But I can't explain tenses in other clauses neither with attracted nor natural sequence of tenses.