Complex sentence: U.S. naval forces had also exchanged gunfire with Iranian gunboats in the fall of 1987, and the U.S. Navy guided-missile frigate had struck an Iranian sea mine in April 1988.
Split into simpler sentences: U.S. naval forces had exchanged gunfire with Iranian gunboats in the fall of 1987. The U.S. Navy guided-missile frigate had struck an Iranian sea mine in April 1988.