@oristarA,
And, what was never before seen, British commanders have extorted victory over the unconquerable valor of our troops by
presenting to the sympathy of their
chief captives awaiting massacre from their savage associates.
I don't really understand "presenting to the sympathy," but I am 100% sure that "chief" here means "main" (it's the only definition for the
adjective chief):
And, what was never before seen, British commanders have extorted victory over the unconquerable valor of our troops by presenting to the sympathy of their
main captives awaiting massacre from their savage associates.