Complex sentence: The poor prospects for success (and possibly his own illness) led Armstrong to abandon his intention of leading the final assault himself, and he returned to Washington on 16 October, just before Wilkinson's part of the campaign was launched.
Split into simpler sentences: The poor prospects for success (and possibly his own illness) led Armstrong to abandon his intention of leading the final assault himself. He handed overall command of the expedition to Wilkinson and returned to Washington on 16 October, just before Wilkinson's part of the campaign was launched.