Complex sentence: He was sent to the US on a Serbian Royal Mission (1917-1919) and was a special delegate to the Yugoslav legation in Washington D.C. (1939-1942) where he died.
Split into simpler sentences: He was sent to the United States on a Serbian Royal Mission from 1917 to 1919. Between 1939 and 1942, he was a special delegate to the Yugoslav legation in Washington D.C., where he died.