Complex sentence: About 1400 merchant ships delivered vital supplies to the Soviet Union under the Lend-Lease program, escorted by ships of the Royal Navy, Royal Canadian Navy, and the U.S. Navy.
Split into simpler sentences: About 1400 merchant ships delivered vital supplies to the Soviet Union under the Lend-Lease program. These were escorted by ships of the Royal Navy, Royal Canadian Navy, the U.S. Navy.