The main landing force for Operation Watchtower was the 1st Marine Division, with reinforcements from the 2nd Marine Regiment, 1st Raider Battalion, 1st Marine Parachute Battalion and 3rd Defense Battalion. The troops that would take part in the landings on Guadalcanal and Tulagi were divided into two Groups, each subdivided into three Combat Groups. The Guadalcanal Group would land on Guadalcanal, while the Northern Group would land on Tulagi and outlying islands. A floating division reserve was also present, consisting of two battalions of the 2nd Marine Regiment and the 3rd Defense Battalion.

Guadalcanal Group consisted of Combat Groups A and B, and a support group. Combat Group A was formed of the 5th Marine Regiment and reinforcing units, less the 2nd Battalion 5th Marines. This group was 3300 men strong. Combat Group B consisted of the 1st Marine Regiment, with reinforcing units, and was 4500 men strong. The Support Group consisted of the 1st Marine Division's communications, engineering, artillery and logistics troops, and was 3500 men strong.

The Northern Group's Combat Groups were designated according to the island they were to take. The Florida Group consisted of the reinforced 1st Battalion 2nd Marines, and was 1300 men strong. The Tulagi Group consisted of the 2nd Battalion 5th Marines, 1st Raider Battalion and elements of the 3rd Defense Battalion. It was the strongest of the Northern Group's forces, at 2200 men. The Gavutu Group was 400 men of the 1st Marine Parachute Battalion.

These troops would have been armed with M1903 Springfields. Each squad would also have had 1-2 BARs, while NCOs, officers and radiomen carried Reising SMGs. Each company had 4 .30 cal light machine guns and 2 60mm mortars in their weapons platoon. The paramarines were armed with Johnson rifles and LMGs.

The main source for the order of battle was John Miller's Guadalcanal: The First Offensive, which can be found online at http://www.history.army.mil/books/wwii/GuadC/GC-fm.htm.