Routes of Allied amphibious forces for landings on Florida, Tulagi, and Gavutu–Tanambogo, 7 August 1942. Source
Japanese forces were well-established in Papua-New Guinea and the Solomon Islands while their ships and aircraft carriers roamed unscathed across vast areas, tapping resources previously exploited by European colonial powers. In early May they seized the southern Solomons and began to build air bases on [Guadalcanal] and the nearby small island of Tulagi. This would have allowed their bombers to attack Allied shipping and support a land-based advance on Port Moresby, the capital city of Papua and the site of an Allied base. The thousands of troops based there were the Allies' last line of defense before Australia.
However, in two great naval battles, the US Navy halted Japan's expansion: in the Coral Sea in May and near Midway Island in June 1942. The time for retreat was over.
The first offensive action by US forces was designed to capture the half-built airfield on Guadalcanal and the sea-plane facility on Tulagi. Men of the 1st Marine Division, who six weeks before had settled in their Paekakariki camps, boarded trains for Queen's Wharf, Wellington. They would join a fleet out of San Diego that carried the 2nd Marine Division, and on one of those ships was Lieutenant Jim Wallace.
Arnold Blumberg, an American historian, explains the context of the experiences recorded in Jim's small black field notebook:
"Having found Tulagi fit only for a seaplane base, on 5 July Japanese forces landed on Guadalcanal, twenty miles across the New Georgia Sound (which Allied servicemen referred to as "The Slot") from Tulagi, and began the rapid construction of Lunga Point Airfield from which the empire's planes could menace the shipping lanes to Australia. In an effort to prevent that eventuality and gain control of the Solomons, 11,000 members of the 1st Marine Division landed at Guadalcanal on 7 August, and captured the airstrip at Lunga Point, as well as the Japanese encampment at Kukum on the west side of Lunga Point the following day. That same afternoon, after fierce fighting, Marines discharged at Tulagi took the Japanese-held Island, as well as the smaller islands of Gavutu and Tanambogo. The captured airstrip on Guadalcanal was renamed Henderson Field, and its occupation and use by Allied forces temporarily halted Japanese expansion in the South Pacific. The significance of American control of the island from which the Allies could expand their presence in the South Pacific while thwarting the Japanese thrust was not lost on the enemy. Guadalcanal became a pivotal piece of island real estate, one that both sides wanted to control and to which they were willing to commit large numbers of forces. By day, aircraft from Henderson Field controlled the skies, allowing U.S. Navy transports and small vessels to operate in the area with some degree of safety. At night, however, control of these waters shifted as Imperial Japanaese Navy warships, then safe from air attack, raced down "The Slot" between the northern and southern Solomons with supplies and troops to resupply Japanese land forces and to assault Allied ships caught outside the protected harbor of the fortified island of Tulagi."