Complex sentence: He began his career as one of the youngest artists to exhibit his paintings at the The Art of This Century Gallery that was opened in New York City during World War II in 1942 by Peggy Guggenheim.
Split into simpler sentences: He began his career as one of the youngest artists to exhibit at the The Art of This Century Gallery. The gallery was opened in New York City during World War II in 1942 by Peggy Guggenheim.