Complex sentence: The German High Command in Paris had no authority over the Nord and Pas-de-Calais "départements", which were instead incorporated into Belgium under the purview of the Military Administration of Belgium and Northern France, based in Brussels.
Split into simpler sentences: The German High Command in Paris had no authority over the Nord and Pas-de-Calais "départements". Instead, they were incorporated into Belgium under the authority of the Military Administration of Belgium and Northern France, based in Brussels.