Complex sentence: This was due to the fact that Lew Ayres, who had played the young doctor beginning in the second movie, was a conscientious objector to World War II. This was an unpopular stance, and he was replaced by actor Keye Luke, portraying a young intern.
Split into simpler sentences: This was due to the fact that Lew Ayres, who had played the young doctor, was a conscientious objector to the war. In the midst of World War II patriotism, Ayres' position became unpopular, and he was replaced by actor Keye Luke as a young intern.