Complex sentence: The U.S. halted Cuban sugar imports, on which Cuba's economy depended the most, and refused to supply its former trading partner with much needed oil, creating a devastating effect on the island's economy.
Split into simpler sentences: The U.S. halted Cuban sugar imports, on which Cuba's economy depended the most. Additionally, the U.S. refused to supply its former trading partner with much needed oil, creating a devastating effect on the island's economy.