[96] See [Appendix G] for a chronological list of all 1st Marine Division operation orders during the Inchon-Seoul operation.
Although CG X Corps was the assigned Expeditionary Troops Commander, planning on the Corps level was concerned almost entirely with the exploitation phase following the seizure of the beachhead. All Landing Force planning was done on the Mount McKinley by the Division in close coordination with PhibGru One.
The first consideration, as viewed by the Navy planners, was that the tides, currents, and tortuous channels of Inchon made necessary a four-hour daylight approach to the transport area. This meant that 1130, at low tide, was the earliest hour of arrival; and not until about 1700 would the next high tide provide enough water for an assault landing.
On 15 September a maximum high tide of 31 feet could be expected at 1919. Evening twilight came at 1909. It was estimated initially that 23 feet of water would take the LCVPs and LVTs over the mud flats, but that 29 feet were necessary for the beaching of the LSTs.
In view of these conditions, PhibGru One planners concluded that 1700 was the best time for landing the LCVPs and LVTs, and it was decided to beach the LSTs at about 1900. Simultaneous landings of troops on Wolmi-do and the mainland were contemplated.
This was the point of departure for Division planners. They maintained that Wolmi-do was the key terrain feature, and that it should be secured first in a separate landing. The logical course, according to the Marines, would be to utilize the morning high tide for the seizure of this island commanding the waterfront. The enemy would be given the whole day in which to prepare for the attack on the mainland; but the Landing Force could also utilize this period for cleaning up Wolmi-do and moving in supporting artillery.
It was typical of the harmony prevailing between the two planning groups on the Mount McKinley that PhibGru One immediately accepted the concept of a double-barreled attack. The rub was that a night approach would be necessary to assault Wolmi-do at 0600 on the morning high tide, and the Navy doubted the feasibility of a movement of the slow-moving and unmaneuverable APAs, AKAs, and LSTs through winding, mud-lined channels in the darkness.
At length a compromise was reached with the decision to employ DD, APD, and LSD types primarily, which were more maneuverable in addition to being equipped with radar navigational instruments.[97]
[97] This summary of Landing Force planning is based upon: 1st MarDiv SAR, basic rpt; MarCorps Board Study, v. I, II-B, 13–16; O. P. Smith, Notes, 58–62; PacFlt Interim Rpt No. 1, XV: Able Able, 6–7.
The morning landing on Wolmi-do was to be made with a single battalion of the 5th Marines, to be designated by the Brigade. On the mainland the remaining two battalions would land with the evening high tide on RED Beach, just north of the causeway connecting the island with the city, while two battalions of the 11th Marines landed in support on Wolmi-do. Meanwhile the 1st Marines was to hit BLUE Beach, southeast of the urban area. And after driving rapidly inland to consolidate their positions before nightfall, the two Marine regiments were to make a junction in the morning and seize the beachhead while the 17th ROK Regiment (later replaced by 1st KMC Regiment) mopped up the city streets.