Complex sentence: In October 1960, a key incident occurred in which a private American oil refinery in Cuba refused to refine a shipment of Soviet crude oil, and the Cuban government responded by nationalizing all three Cuban refineries, which were all American-owned.
Split into simpler sentences: In October 1960, a key incident occurred in which a private American oil refinery in Cuba refused to refine a shipment of Soviet crude oil. The Cuban government responded by nationalizing all three Cuban refineries, which were all American-owned.