At the same time as major battles raged in New Guinea, Allied forces became aware of a Japanese airfield under construction at Guadalcanal through coastwatchers. On 7 August 1942, US Marines landed on the islands of Guadalcanal and Tulagi in the Solomons. Vice Admiral Gunichi Mikawa, commander of the newly formed Eighth Fleet at Rabaul, reacted quickly. Gathering five heavy cruisers, two light cruisers, and a destroyer, he sailed to engage the Allied force off the coast of Guadalcanal. On the night of 8–9 August, Mikawa's quick response resulted in the Battle of Savo Island, a brilliant Japanese victory during which four Allied heavy cruisers were sunk, while no Japanese ships were lost. It was one of the worst Allied naval defeats of the war. The victory was mitigated only by the failure of the Japanese to attack the vulnerable transports. Had it been done so, the first American counterattack in the Pacific could have been stopped. The Japanese originally perceived the American landings as nothing more than a reconnaissance in force.