Passage: As the transports unloaded on the night of 8–9 August, two groups of screening Allied cruisers and destroyers, under the command of British Rear Admiral Victor Crutchley VC, were surprised and defeated by a Japanese force of seven cruisers and one destroyer from the 8th Fleet based at Rabaul and Kavieng and commanded by Japanese Vice Admiral Gunichi Mikawa. In the Battle of Savo Island one Australian and three American cruisers were sunk and one American cruiser and two destroyers were damaged. The Japanese suffered moderate damage to one cruiser. Mikawa, who was unaware Fletcher was preparing to withdraw with the U.S. carriers, immediately retired to Rabaul without attempting to attack the transports. Mikawa was concerned about daylight U.S. carrier air attacks if he remained in the area. Bereft of his carrier air cover, Turner decided to withdraw his remaining naval forces by the evening of 9 August and in so doing left the Marines ashore without much of the heavy equipment, provisions and troops still aboard the transports. Mikawa's decision not to attempt to destroy the Allied transport ships when he had the opportunity proved to be a crucial strategic mistake.

Question: What great mistake did Mikawa did?
decision not to attempt to destroy the Allied transport ships