Complex sentence: The U.S. vessels started firing at the four ships from a distance of, which was accurate at first, until the four Spanish boats backed up to of the U.S. ships, and, at 1:30 PM, the U.S. ships entered the harbor.
Split into simpler sentences: The U.S. vessels started firing at the four ships from a distance of. The fire was accurate at first, until the four Spanish boats backed up to of the U.S. ships, and, at 1:30 PM, the U.S. ships entered the harbor.