Complex sentence: Winston Churchill personally chose him to lead a liaison mission to central Yugoslavia in 1943, where Josip Broz Tito and his partisans were emerging as a major irritant to the German control of the Balkans.
Split into simpler sentences: Winston Churchill personally chose him to lead a liaison mission to central Yugoslavia in 1943. At the time of MacLean's deployment to Yugoslavia, Josip Broz Tito and his partisans were emerging as a major irritant to the German control of the Balkans.