The Battle of Mount Austen, the Galloping Horse, and the Sea Horse was fought from December 1942 to January 1943 during the Guadalcanal campaign of World War II.
Throughout the campaign, the Japanese used Mount Austen to observe the American defenses around the Henderson Field airbase at Lunga Point. Emplaced artillery on Mount Austen harassed Henderson Field and protected the Japanese positions around the upper Matanikau valley, while also protecting the Maruyama Road reinforcement route. After the Battle for Henderson Field, General Harukichi Hyakutake reinforced the soldiers and artillery emplaced on the ridgeline to prepare for the next planned attack on the Americans. Colonel Akinosuka Oka's IJA 124th Infantry Regiment and remnants of the IJA 230th Infantry Regiment took up defensive positions along Mount Austen, and they were soon joined by the IJA 228th Infantry Regiment and a battalion of the IJA 229th Infantry Regiment, landed by the Tokyo Express.
Meanwhile, the US Army reinforced its own forces on Guadalcanal, transferring three infantry regiments of the Americal Division to the island. On 9 December 1942, Major General Alexander Patch replaced the US Marine Corps forces on Guadalcanal with fresh Army soldiers who were ordered to eliminate all Japanese forces remaining on Guadalcanal. The US 25th Infantry Division was soon sent in to reinforce Patch for this purpose.
On 16 December 1942, in response to Japanese infiltration raids on Henderson Field, Patch ordered the US 132nd Infantry Regiment to secure Mount Austen. The Japanese defenders pinned down the Americans with machine-gun and rifle fire on 18 December. From 20 to 23 December, the Japanese withdrew to Hill 31, and the Americans soon came up against the Gifu position, the most strongly fortified Japanese position on the island. Camouflaged pillboxes manned by 1-2 machine guns and several riflemen provided mutual support to each other. From 25 to 29 December, the Japanese fended off American attacks on the position. By 29 December, the Americans had lost 53 killed, 129 wounded, and 131 sick. On 3 January 1943, the Americans finally occupied the summit of Hill 27, and they pushed towards the Gifu, whose defenders had consumed their last remaining food rations on 1 January. The Americans took Mount Austen with a total loss of 115 killed and 272 wounded; 500 Japanese had been killed and wounded.
On 5 January 1943, Patch issued his plan to begin clearing Guadalcanal of Japanese forces. The 25th Division was sent in to finish clearing Mount Austen and secure the hilltops and ridges located along the Matanikau. The Americans came up against the Gifu, the Galloping Horse ridge, and the two hills of the Sea Horse, facing fierce resistance. By 13 January, the 2nd Battalion had suffered 75% losses. The capture of the Sea Horse ultimately isolated the Gifu from the rest of the IJA 17th Army, and the Japanese commander Major Takeyoshi Inagaki refused to abandon his sick and wounded and insisted on fighting to the last. On 17 January, the Americans launched a massive artillery bombardment against the Gifu, and they destroyed several pillboxes from 18 to 20 January before heavy rain stopped the attack. On 23 January, Inagaki and his 100 remaining soldiers launched a suicidal banzai charge against the Americans, fighting to the last man. Meanwhile, the 2nd Marine Division drove up the Guadalcanal coast, and 643 of Lieutenant General Masao Maruyama's Japanese soldiers were killed.
On 18 January, Hyakutake reluctantly ordered the evacuation of Guadalcanal. The Japanese fought rearguard actions against the advancing Americans, evacuating 10,652 troops from the island from 1 to 7 February. On 9 February 1943, the Americans declared Guadalcanal secure.
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