Complex sentence: Before the rise of the NSDAP it was the main nationalist party in Weimar Germany composed of nationalists, reactionary monarchists, völkisch, and antisemitic elements, and supported by the Pan-German League.
Split into simpler sentences: Before the rise of the NSDAP it was the major conservative and nationalist party in Weimar Germany. It was an alliance of nationalists, reactionary monarchists, völkisch, and antisemitic elements, and supported by the Pan-German League.