With southern and eastern Peleliu captured, there now began an encirclement of the Japanese defenders in central Peleliu, and an attack against the Japanese defending northern Peleliu and nearby Ngesebus and Kongauru. This was the obvious next tactical phase for combat on Peleliu. However, securing it was less necessary for the basic Peleliu tactical and strategic goals than for the mopping-up of the island. As the 1st Marine Division’s Assistant Commander, Brigadier General Oliver P. Smith, later phrased it, “by the end of the first week, the Division had control of everything on the island that it then needed, or later used.”

The airfield had been seized, was under repair and improvement, and in use. It was no longer any threat, if it had ever been, to MacArthur’s long-heralded return to the Philippines. Peleliu’s best logistical beach (Purple) had been secured, providing a secure logistic axis to the main battle areas. The Japanese defenders in their caves, and in northern Peleliu and on Ngesebus, retained some capability to harass American rear installations, but that was sharply curtailed by the Marines’ counterfire.

Only two significant Japanese capabilities remained: they could bitterly resist from their cave positions and they had a limited capability to reinforce Peleliu from Babelthuap. Such reinforcement could only be by small-unit infiltration, which faced U.S. naval screening operations in the area. Likewise, American encirclement of the stubborn Umurbrogol Pocket faced two obstacles. First was the lack of additional maneuver regiments from III Amphibious Corps’ reserve. General Geiger in fact had no corps reserve pending the release of some units from the forces involved in the seizure of Angaur. That landing by the 81st Infantry Division (less the 323d RCT) had been launched on 17 September, after which there was no corps reserve.

The operation on Angaur, the planning which attended it and the decision on its timing, impacted heavily upon the Peleliu operation. The naval planners early on proposed landing on Angaur before Peleliu. Only when Major General Julian C. Smith, commanding Expeditionary Troops/X-Ray Planning Group, explained that such timing would invite the numerous Japanese in northern Palau to reinforce Peleliu was it agreed that Angaur be assaulted only after the Peleliu landing was assured of success. However, the Angaur landing was initiated before the Peleliu landing had been clearly resolved. The commanding general of the 81st Division wanted to land as soon as possible, and he was supported in his view by his naval task unit commander, Rear Admiral William H.P. Blandy. Opposing the 17 September date for the Angaur landing was Marine Major General Julian Smith. Smith argued that committing the element of III Corps Reserve before the Peleliu operation was more fully developed would be premature. His advice was ignored by Vice Admiral Theodore S. Wilkinson.

A related decision on 17 September committed the III Corps’ final reserve to the Ulithi landing. The task was assigned to the Western Attack Force, which was ordered to seize Ulithi with “available resources.” Over General Smith’s advice, Wilkinson chose to commit the entire 323d RCT, the 81st Division’s other maneuver element. The 321st subsequently and successfully occupied an undefended Ulithi while reserves were sorely needed at Peleliu.

By 20 September, the 81st Division had defeated or cornered all survivors of Angaur’s 1,400 defenders. The 81st’s commander declared Angaur secure. He tasked his 322d RCT to complete the mop-up, and reported to General Geiger that the 321st RCT was available for further operations. The lack of enough troops to begin encircling Umurbrogol was no longer an obstacle.

The other obstacle to reinforcing the division on Peleliu and encircling the Pocket lay in the thinking of General Rupertus, who clung to a belief that his Marines could do it without help from the Army. The III Corps plan tasked the 81st Division to reinforce the Marines in seizing Peleliu and then to relieve the 1st Marine Division for the mop up, but the general continued to exhort his commanders to “hurry up.”

Earlier, General Rupertus and Colonel Puller had shrugged off a suggestion from the 5th Marines’ “Bucky” Harris that they take a look at the Umurbrogol Pocket from the newly available light planes of Marine Observation Squadron 3. Harris’ own aerial reconnaissance, made immediately after those planes arrived on 19 September, had altered his view of the Umurbrogol from sober to grave. It convinced him that attacking the Pocket from the north would be less costly than the originally planned and ordered attempts from south to north. Both Puller and Rupertus responded to Harris that they had their maps.

The prelanding scheme of maneuver was built on the tactical concept that, after capturing the airfield, the 1st Marine Division would push north along a line across the width of the main or western part of the island. Once abreast of the southern edge of Umurbrogol, that concept and maneuver scheme were reflected in a series of four west-to-east phase lines, indicating an expected linear advance, south to north. Clearly, it was expected that the advance along the flatter zones west and east of Umurbrogol would be at approximately the same pace as that along the high-central ground of Peleliu. Such thinking may have been consistent with Rupertus’ prediction of a three-day assault. Developments in Sabol’s sector to the west, and in the 5th Marines’ sector to the east, apparently did not change division-level thinking. Until additional forces became available, such a linear advance may have seemed all that was possible.

Even so, there was no apparent reexamination of the planned south-to-north linear advance, and for days after the Pocket was sealed off at its northernmost extremity, the division commander kept ordering attacks from south to north, generally following the initial landing plan. As had been revealed to “Bucky” Harris in his early aerial reconnaissance of the Umurbrogol Pocket, such attacks would offer little but casualties. Troops, heavily supported, could advance into “the Horseshoe” and into “Death Valley,” but the positions they reached then proved untenable and withdrawal was usual at day’s end.

Some part of this thinking may have well come from the inadequacies of the map in use. The 5th Marines in early October produced a new and more representative sketch map. It located and identified the details within Umurbrogol sufficiently to facilitate maneuver and fire coordination.

That mapping effort, incidentally, led to the misnaming of Honsowetz’ Hill 100, where Captain Everett P. Pope earned his Medal of Honor. The 5th’s mapping team, launched after Harris’ regiment was committed against the Pocket, encountered Lieutenant Colonel Walt, the regimental executive officer, on Hill 100 during their sketching, and so named the hill.

Even after General Geiger had ordered General Rupertus on 21 September to stand down Puller’s shattered 1st Marines, General Rupertus expressed the belief that his Marines, alone, would shortly clear the entire island. After taking a closer look at the situation on the ground, Geiger ordered RCT 321 from Angaur and attached it to the Marine division. Encirclement of the Umurbrogol Pocket now became tactically feasible.

Capture of northern Peleliu and Ngesebus became more pressing with the discovery on 23 September that some part of the enemy’s substantial troop strength in the northern Palaus was being infiltrated by barge from Koror and Babelthuap into northern Peleliu.

Although the naval patrol set to protect against just that reinforcing action had discovered and destroyed some of the Japanese barges, most enemy troops seemed to have waded ashore on the early morning of 23 September. Colonel Nakagawa suddenly had reinforcements in the form of a partially mauled infantry battalion in northern Peleliu.