Complex sentence: Rafael Leónidas Trujillo came to power in 1930 in the neighboring Dominican Republic, and in 1937, he attacked the border with Haiti, killing an estimated 20,000 Haitians.
Split into simpler sentences: Rafael Leónidas Trujillo had come to power in 1930 in the neighboring Dominican Republic. In 1937, Trujillo attacked the border with Haiti, his forces killing an estimated 20,000 Haitians.