Complex sentence: The MNR was founded in 1940 by former members of the fiercy pro-Catholic and authoritarian Rexist Party who were disillusioned by the cooperation of the party with German occupying forces, including Eugène Mertens de Wilmars, a former admirer of Leon Degrelle.
Split into simpler sentences: The MNR was founded in 1940 by former members of the fiercy pro-Catholic and authoritarian Rexist Party who were disillusioned by Rex's policy of collaboration with the German occupying forces. These including Eugène Mertens de Wilmars, a former admirer of Leon Degrelle.