Complex sentence: Von Oppenheim was a controversial figure before and during World War I because he was considered a spy by the French and British and did in fact engage in Anti-Allies propaganda, aimed at stirring up the Muslim populations of the Allied-controlled territories against their colonial masters.
Split into simpler sentences: Von Oppenheim was a controversial figure before and during World War I because he was considered a spy by the French and British. He did in fact engage in Anti-Allies propaganda, aimed at stirring up the Muslim populations of the Allied-controlled territories against their colonial masters.