Complex sentence: In September 1943, after receiving word of his impending arrest by the Germans, he fled to Sweden, then to Britain, where he joined the British Tube Alloys nuclear weapons project, and later the Manhattan Project.
Split into simpler sentences: In September 1943, after receiving word of his impending arrest by the Germans, he fled to Sweden. Flown next to Britain, he joined the British Tube Alloys nuclear weapons project, and later the Manhattan Project.