Complex sentence: During the 1910 to 1945 colonial period, approximately 2.4 million ethnic Koreans immigrated to Japan for economic reasons, though some brought over forcibly during the Second World War to work as laborers.
Split into simpler sentences: During the 1910 to 1945 colonial period, approximately 2.4 million ethnic Koreans immigrated to Japan. Some for economic reasons, and some were forced to move during the Second World War to work as laborers.