Over the next several months, the Japanese established a naval refueling, communications, and seaplane reconnaissance base on Tulagi and the nearby islets of Gavutu and Tanambogo, and in July 1942 began to build a large airfield on nearby Guadalcanal. The Japanese activities on Tulagi and Guadalcanal were observed by Allied reconnaissance aircraft, as well as by Australian coastwatcher personnel stationed in the area. Because these activities threatened the Allied supply and communication lines in the South Pacific, Allied forces counter-attacked with landings of their own on Guadalcanal and Tulagi on 7 August 1942, initiating the critical Guadalcanal campaign and a series of combined arms battles between Allied and Japanese forces that, along with the New Guinea campaign, decided the course of the war in the South Pacific.