Allied warships bombed the invasion beaches while U.S. carrier aircraft bombed Japanese forces on the target islands and destroyed 15 Japanese seaplanes at their base near Tulagi.
Tulagi and two nearby small islands, Gavutu and Tanambogo, were attacked by 3,000 U.S. Marines. The 886 IJN forces violently resisted the Marine attacks. With some difficulty, the Marines captured all three islands; Tulagi on 8 August, and Gavutu and Tanambogo by 9 August.
The Japanese defenders were almost all killed while the Marines had 122 killed.
The landings on Guadalcanal had much less resistance. On 7 August, 11,000 U.S. Marines came ashore on Guadalcanal and met no resistance. They stopped for the night about from the Lunga Point airfield. The next day, against little resistance, the Marines advanced to Lunga River and captured the airfield by 16:00 on 8 August.
The Japanese naval construction staff and combat troops had abandoned the airfield area and fled about west to the Matanikau River and Point Cruz area. They left behind food, supplies, construction equipment and vehicles, and 13 dead.
During the landing operations on 7 and 8 August, Japanese naval aircraft based at Rabaul, under the command of Sadayoshi Yamada, attacked the Allied forces several times, setting afire the transport USS "George F. Elliot" (which sank two days later) and heavily damaging the destroyer . In the air attacks over the two days, the Japanese lost 36 aircraft, while the U.S. lost 19, both in combat and to accident, including 14 carrier fighters.
After these fights, Fletcher was concerned about the losses to his carrier fighter aircraft. He was also anxious about the threat to his carriers from Japanese air attacks, and worried about his ships' fuel levels. Fletcher pulled back from the Solomon Islands area with his carrier task forces the evening of 8 August. As a result of the loss of carrier-based air cover, Turner pulled back his ships from Guadalcanal, even though less than half of the supplies and heavy equipment needed by the troops ashore had been unloaded. Turner planned, however, to unload as many supplies as possible on Guadalcanal and Tulagi throughout the night of 8 August and then depart with his ships early on 9 August.
That night, as the transports unloaded, two groups of Allied cruisers and destroyers, under the command of British Rear Admiral Victor Crutchley VC, were defeated by a Japanese force of seven cruisers and one destroyer from the 8th Fleet based at Rabaul and Kavieng and commanded by Japanese Vice Admiral Gunichi Mikawa.
In the Battle of Savo Island one Australian and three American cruisers were sunk and one American cruiser and two destroyers were damaged. The Japanese had moderate damage to one cruiser. Mikawa, who was unaware Fletcher was preparing to pull back with the U.S. carriers, immediately went back to Rabaul without attempting to attack the transports. Mikawa was concerned about daylight U.S. carrier air attacks if he remained in the area. Without carrier air cover, Turner decided to pull back his remaining naval forces by the evening of 9 August. This left the Marines ashore without much of the heavy equipment, supplies and troops still aboard the transports. Mikawa's decision not to attempt to destroy the Allied transport ships was a big mistake.
The 11,000 Marines on Guadalcanal at first set up a guard around Lunga Point and the airfield. They also moved the landed supplies ashore and finished the airfield. By 18 August the airfield was ready for operation. Five days worth of food had been landed from the transports, which, along with captured Japanese food, gave the Marines a total of 14 days worth of food. To conserve supplies, the troops were limited to two meals per day.
Allied troops got sick with dysentery soon after the landings, with one in five Marines sick by mid-August. Tropical diseases affected both sides' troops. Although some of the Korean construction workers surrendered to the Marines, most of the Japanese and Korean gathered to the west and ate coconuts. A Japanese naval outpost was also located at Taivu Point, about 35 kilometres (22 mi) east of the Lunga perimeter. On 8 August, a Japanese destroyer from Rabaul delivered 113 troops to the Matanikau position.
On the evening of 12 August, a 25-man U.S. Marine patrol landed by boat to try and find a group of Japanese troops that U.S. forces thought might be willing to surrender. Soon after the patrol landed, a nearby platoon of Japanese naval troops attacked and almost completely killed the Marine patrol.
On 19 August, Vandegrift sent three companies of the U.S. 5th Marine Regiment to attack the Japanese troops west of the Matanikau. One company attacked at the mouth of the Matanikau river while another crossed the river inland and attacked the Japanese forces in Matanikau village. The third landed by boat further west and attacked Kokumbuna village. After briefly occupying the two villages, the three Marine companies returned to the Lunga perimeter, having killed about 65 Japanese soldiers while losing four.
This was the first of several major actions around the Matanikau River during the battle.
On 20 August, the escort carrier delivered two squadrons of Marine aircraft to Henderson Field, one a squadron of 19 F4F Wildcats and the other a squadron of 12 SBD Dauntlesses. The aircraft at Henderson became known as the "Cactus Air Force" (CAF) after the Allied codename for Guadalcanal.
The Marine fighters were used the next day. There were Japanese bomber air raids almost every day. On 22 August five U.S. Army P-400 Airacobras and their pilots arrived at Henderson Field.
In response to the Allied landings on Guadalcanal, the Japanese Imperial General Headquarters assigned the Imperial Japanese Army's (IJA) 17th Army at Rabaul and under the command of Lieutenant General Harukichi Hyakutake, was ordered to retake Guadalcanal. The army was to be supported by Japanese naval units, including the Combined Fleet under the command of Isoroku Yamamoto, which was headquartered at Truk. The 17th Army, at that time involved in the Japanese campaign in New Guinea, had only a few units available. The 35th Infantry Brigade under Major General Kiyotake Kawaguchi was at Palau. The 4th (Aoba) Infantry Regiment was in the Philippines and the 28th (Ichiki) Infantry Regiment, under the command of Colonel Kiyonao Ichiki, was on transport ships near Guam.
The different units began to move towards Guadalcanal. Ichiki's unit, consisting of about 917 soldiers, landed from destroyers at Taivu Pointbafter midnight on 19 August, then made a night march west toward the Marines. The Ichiki regiment was named after its commanding officer.
Ichiki thought that there were not that many Allied soldiers on Guadalcanal. Ichiki's soldiers attacked Marine positions at Alligator Creek in the early morning hours of 21 August. Ichiki's attack was defeated with heavy Japanese losses in what became known as the Battle of the Tenaru. In the day, the Marines attacked Ichiki's surviving troops, killing many more of them.
The dead included Ichiki, though it has been claimed that he committed suicide after realizing his defeat, rather than dying in combat. In total, all but 128 of the original 917 members of the Ichiki Regiment's force were killed in the battle. The survivors returned to Taivu Point, notified 17th Army headquarters of their defeat and waited for new soldiers and orders from Rabaul.
As the Tenaru battle was ending, more Japanese troops were already on their way. Three slow transports departed from Truk on 16 August carrying the remaining 1,400 soldiers from Ichiki's (28th) Infantry Regiment plus 500 naval marines from the 5th Yokosuka Special Naval Landing Force.