Complex sentence: The underlying cause of the riots was the deteriorating economic conditions facing Puerto Ricans and incoming African Americans embodied many aspects of the national urban crisis while ethnic conflicts, especially those between young Puerto Ricans and Polish Americans, prevailed during the transition period.
Split into simpler sentences: The underlying cause of the riots was the deteriorating economic conditions facing Puerto Ricans and African Americans, which embodied many aspects of the national urban crisis. Ethnic conflicts, especially those between young Puerto Ricans and Polish Americans, prevailed during the transition period.