Complex sentence: On 5 November, however, two U.S. aircraft carriers raided Rabaul, heavily damaging four heavy cruisers and forcing them to retreat back to Truk, ending the Japanese warship threat to the Allied landing forces at Bougainville.
Split into simpler sentences: On 5 November, however, two U.S. aircraft carriers raided Rabaul, heavily damaging four heavy cruisers, which had to withdraw to Truk. This ended the Japanese warship threat to the Allied landing forces at Bougainville.