Five Marine battalions cross the Matanikau in a pincer; the Japanese line east of the river breaks
Nine weeks after landing on Guadalcanal, the Marines still hadn't secured the ground west of their own perimeter, and Japanese troops were massing along the Matanikau River to prepare gun positions aimed at Henderson Field. On October 6, 1942, two battalions of the 5th Marine Regiment crossed at the river's mouth while three more battalions, with Colonel William Whaling's scouts and snipers, crossed inland at a single-log footbridge to close the trap from behind. The pincer forced Japanese units under Major General Yumio Nasu back from their positions east of the river, inflicting heavy losses. The fighting continued through October 9, pushing the Japanese line west of the river.
The October attack fixed what the September attack had gotten wrong. Two weeks earlier, an uncoordinated attack at the same river had been repulsed; this time division headquarters kept the whole operation under one command with artillery and air support planned in advance. The coordinated approach held through the harder fighting still ahead for Henderson Field, and it became a case the Marine Corps taught for generations as a lesson in coordinated planning.
QWhy did Marines on a ridge resort to spelling out messages with their own undershirts?
During the September fighting that preceded the October crossing, three Marine companies got surrounded by Japanese forces on a rise known as Hill 84 and lost radio contact with the rest of the regiment. Cut off and taking losses, the Marines stripped off their white undershirts and laid them out on the ridge to spell H-E-L-P, hoping someone overhead would notice. A Cactus Air Force SBD Dauntless flying support for the operation spotted the message and relayed it back to command. The trapped Marines eventually escaped only with help from a Navy destroyer and landing craft crewed by Coast Guardsmen. The near-disaster convinced Vandegrift's staff the September plan needed a total rework before trying the river again.
QWhat specific Japanese units landed on Guadalcanal just two days before the first failed Matanikau attack?
On 11 September, the 3rd Battalion of the 4th Aoba Infantry Regiment came ashore at Kamimbo Bay on Guadalcanal's western tip, arriving too late to take part in Kawaguchi's assault on the Marine ridge that began the next day. Rather than sitting idle, the battalion folded into Colonel Akinosuke Oka's force already positioned near the Matanikau. Tokyo Express runs kept feeding the buildup afterward, delivering food, ammunition, and 280 more men from the 1st Battalion, Aoba Regiment starting 15 September. These late arrivals quietly thickened the Japanese line west of the river well before Maruyama's October reinforcements ever showed up.
QHow many Marines arrived in the single convoy that let Vandegrift finally close his defensive ring around Lunga Point?
On 18 September, an Allied convoy delivered 4,157 men of the 3rd Provisional Marine Brigade to Guadalcanal, a single infusion large enough to let Vandegrift close his defensive ring and free battalions for offense. Vandegrift used the fresh troops to do something he couldn't manage before: starting 19 September, he established an unbroken defensive line completely around the Lunga perimeter for the first time since the landings. That solid ring gave the Marines the manpower to spare battalions for offensive patrols into the Matanikau Valley rather than holding every man back purely for defense, a shift that set up both the September raid and the better-resourced October crossing that followed it.
