In the early 20th century, several British and Australian firms began large-scale coconut planting. Economic growth was slow, however, and the islanders benefited little. With the outbreak of World War II, most planters and traders were evacuated to Australia, and most cultivation ceased. Some of the most intense fighting of World War II occurred in the Solomons.

The most significant of the Allied Forces' operations against the Japanese Imperial Forces was launched on August 7, 1942 with simultaneous naval bombardments and amphibious landings on the Florida Islands at Tulagi and Red Beach on Guadalcanal. The Battle of Guadalcanal became an important and bloody campaign fought in the Pacific War as the Allies began to repulse Japanese expansion. Of strategic importance during the war were the coastwatchers operating in remote locations, often on Japanese held islands, providing early warning and intelligence of Japanese naval, army and aircraft movements during the campaign.

Sergeant-Major Jacob Vouza was a notable coastwatcher who after capture refused to divulge Allied information in spite of interrogation and torture by Japanese Imperial forces. He was awarded the highest award for bravery by the Americans. Islanders Biuku Gasa and Eroni Kumana would be noted by National Geographic for being the first to find the shipwrecked John F. Kennedy and his crew of the PT-109. They suggested using a coconut to write a rescue message for delivery by dugout canoe, which was later kept on his desk when he became the president of the United States.

The Solomon Islands was one of the major staging areas of the South Pacific and was home to the legendary VMF-214 "Black Sheep" Squadron commanded by Major Greg "Pappy" Boyington. The group of islands was part of what was called the Slot.