The next day Red Mike discussed the situation with division planners. Intelligence officers translating the captured documents confirmed that 3,000 Japanese were cutting their way through the jungle southwest of Tasimboko. Edson was convinced that they planned to attack the currently unguarded southern portion of the perimeter. From an aerial photograph he picked out a grass-covered ridge that pointed like a knife at the airfield. His hunch was based on his own experience in jungle fighting and with the Japanese. He knew they liked to attack at night, and that was also the only time they could get fire support from the sea. And a night attack in the jungle only had a chance if it moved along a well-defined avenue of approach. The ridge was the obvious choice. Thomas agreed. Vandegrift did not, but they convinced the general to let the raiders and parachutists shift their bivouac to the ridge in order to get out of the pattern of bombs falling around the airfield.
The men moved to the new location on 10 September. Contrary to their hopes, it was not a rest zone. Japanese planes bombed the ridge on the 11th and 12th. Native scouts brought reports of the approaching enemy column, and raider patrols soon made contact with the advance elements of the force. The Marines worked to improve their position under severe handicaps. There was very little barbed wire and no sandbags or engineering tools. Troops on the ridge itself could not dig far before striking coral; those on either flank were hampered by thick jungle that would conceal the movement of the enemy. Casualties had thinned ranks, while illness and a lack of good food had sapped the strength of those still on the lines.
Edson and Thomas did the best they could with the resources available. Red Mike used the spine of the ridge as the dividing point between his two rump battalions. One company of parachutists held the left of his line, with the rest of their comrades echeloned to the rear to protect that flank. Two companies of raiders occupied the right, with that flank anchored on the Lunga River. A lagoon separated the two raider units. Edson attached the machine guns to the forward companies and kept the remaining raiders in reserve. (Cornpany D was no larger than a platoon now, since Red Mike had used much of its manpower to fill holes in the other three rifle companies.) He set up his forward command post on Hill 120, just a few hundred yards behind the front lines.
Thomas placed the 2d Battalion, 5th Marines, in reserve between the ridge and the airfield. Artillery forward observers joined Edson and registered the howitzers. The Marines were as ready as they could be, but the selection of the ridge as the heart of the defense was a gamble. To the west of the Lunga there were only a few strongpoints occupied by the men from the pioneer and amphibious tractor battalions. To the east of Red Mike’s line there was nothing but a mile of empty jungle.
The Japanese commander Kawaguchi was having his own problems. In addition to the setback at Tasimboko, his troops were having a tough time cutting their way through the heavy jungle and toiling over the many ridges in their path. Some of his difficulties were self-inflicted. His decision to attack from the south had required him to leave his artillery and most of his supplies behind, since they could not be hauled over the rough jungle trail. Thus he would go into battle with little fire support and poor logistics. He then detailed one of his four battalions to make a diversionary attack along the Tenaru. This left him with just 2,500 men for the main assault. Finally, he had underestimated the time needed to reach his objective.
On the evening of 12 September, as the appointed hour for the attack approached, Kawaguchi realized that only one battalion had reached its assigned jump-off point, and no units had been able to reconnoiter the area of the ridge. He wanted to delay the attack, but communications failed and he could not pass the order. Behind schedule and without guides, the battalions hastily blundered forward, only to break up into small groups as the men fought their way through the tangled growth in total darkness. At 2200 a Japanese plane dropped a series of green flares over the Marine perimeter. Then a cruiser and three destroyers opened up on the ridge. For the next 20 minutes they poured shells in that direction, though most rounds sailed over the high ground to land in the jungle beyond, some to explode among the Japanese infantry.
When the bombardment ceased, Kawaguchi’s units launched their own flares and the first piecemeal attacks began. The initial assault concentrated in the low ground around the lagoon. This may have been an attempt to find the American flank, or the result of lack of familiarity with the terrain. In any case, the thick jungle offset the Marine advantage in firepower, and the Japanese found plenty of room to infiltrate between platoon strongpoints.