Complex sentence: In the mid 19th to 20th centuries, hundreds of thousands of Chinese men in the U.S, mostly of Cantonese origin from Taishan, migrated to the United States.
Split into simpler sentences: In the mid 19th to 20th centuries, the Chinese that migrated were almost entirely of Cantonese origin. Hundreds of thousands of Chinese men in the U.S, mostly of Cantonese origin from Taishan migrated to the United States.