Complex sentence: Contemporaries concluded that the Muslim threat to Ethiopia was finally over, so when the diplomatic mission from Portugal arrived at last, Dawit denied that Mateus had the authority to negotiate treaties, ignoring Eleni's counsels.
Split into simpler sentences: Contemporaries concluded that the Muslim threat to Ethiopia was finally over. When the diplomatic mission arrived from Portugal, Dawit downplayed the authority that Mateus had to negotiate treaties, ignoring Eleni's counsels.