Complex sentence: ONUC's initial mandate, however, only covered peacekeeping and Hammarskjöld viewed the secessions as an internal political matter and that opposing them would represent a loss of neutrality and a breach of Congolese sovereignty.
Split into simpler sentences: ONUC's initial mandate, however, only covered peacekeeping. Viewing the secessions as an internal political matter, Hammarskjöld refused to use UN troops to assist the central Congolese government against them; he argued that doing so would represent a loss of impartiality and breach Congolese sovereignty.