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Army planning had been initiated by the Joint Strategic Plans and Operations Group until 16 August, when the “Special Planning Staff” was set up at GHQ to issue directives for Operation Plan CHROMITE.
Published on 12 August as CinCFE Operation Plan No. 100–B, it was based on these assumptions:
(a) that the North Korean ground advance would be stopped in time to permit the build-up of our forces in South Korea;
(b) that our forces in South Korea would be built up to the capability of mounting effective offensive operations against NKPA forces opposing them;
(c) that we retain air and naval supremacy in the area of operation;
(d) that the NKPA ground forces would not receive major reinforcements from the USSR or Red China;
(e) that there would be no major change in the basic disposition of the NKPA forces.
It was understood from the beginning that the Special Plans Staff, headed by General Ruffner, would be the nucleus of the future X Corps staff. In order to have the benefit of specialized amphibious knowledge, ten Marine and two Navy officers of TTU Mobile Training Team Able were assigned on 19 August:
These officers did not begin their new assignment in time to contribute to the preliminary X Corps overall scheme of maneuver. The main provisions, as communicated to General Smith at General Ruffner’s briefing conference of 23 August, were as follows:
(1) The 1st Marine Division, as the landing force, was to seize the urban area of Inch'ŏn (line A–A); to capture a beachhead (line B–B); to advance as rapidly as possible and seize Kimp'o Airfield (line C–C); to clear out the south bank of the Han River (line D–D); to cross the river, seize Sŏul and secure the commanding ground to the north (E–E); and, finally, to fortify and occupy this line with reduced forces until relieved (apparently by the 3rd Infantry Division, still in the United States), whereupon the Division was to re-cross the Han and seize a line (F–F) about 25 miles southeast of Sŏul.
(2) The 7th Infantry Division was to land behind the Marines and advance on their right flank to seize the commanding ground south of Sŏul and the south bank of the river (line D–D); to continue the advance to phase line (E–E); and to conduct a reconnaissance in force to the south (line F–F). There, on the line from Suwŏn to Kyŏngsan-ni, the 7th Infantry Division and 1st Marine Division would form the strategic anvil as Eighth Army forces advanced from the Pusan Perimeter in the role of hammer.
(3) The 1st Marine Aircraft Wing was to furnish air support, air direction, and air warning for the Corps with units operating from Kimp'o Airfield. It was also to be prepared to operate a control center ashore on order. 
The Special Plans Staff gave General Smith a study explaining the purposes of these maneuvers. “The B– B line in this study appeared to be a suitable beachhead line,” he commented, “and we decided to concentrate our efforts on plans for its seizure. Subsequent operations would be reserved for later consideration.”
Good planning, of course, depended on accurate intelligence. All possible information about the objective area had been gathered by the staff of PhibGru One before the arrival of the 1st Marine Division planners. Air Force planes had taken hundreds of photographs at every stage of the tide. Hydrographic reports and navigation charts had been studied. Army and Navy men familiar with Inch'ŏn during the American occupation after World War II were interrogated as well as NKPA prisoners captured by the Eighth Army. Although a great deal of useful data was compiled, some disturbing questions remained. How high were the sea walls of Inch'ŏn? Were the mud flats suitable for landing either troops or vehicles at low tide? Approximately how many NKPA guns were hidden on Wŏlmi-do?… These were some of the intelligence gaps which must be filled before an effective plan could be drawn up for an assault landing.
PhibGru One made its material available to the G–2 Section of the 1st Marine Division, and the two staffs worked together on the SS Mount McKinley (AGC-7) in close cooperation. Attached were the 163rd Military Intelligence Service Detachment (MISD) and the 441st Counter Intelligence Corps (CIC) Team. Both of these units had been furnished by FECom and consisted of Army commissioned and enlisted personnel as well as native Koreans serving in liaison, interpretation, and translation capacities.
Even when a question could not be answered conclusively, it was up to the G–2 sections of the Attack Force and Landing Force to arrive at a conclusion for planning purposes. For instance, it was never satisfactorily determined from available sources—JANIS publications, strategic engineering studies, Naval Attaché reports, and photographic interpretation reports—whether LVTs would be able to traverse the mud flats of the Inch'ŏn harbor area. And since there remained some doubt, planning proceeded on the assumption that the answer was negative. This proved to be the correct as well as the prudent decision, later developments revealed.
Another G–2 planning problem concerned the effect that the height of the sea walls would have upon the landing. Photographs at hourly stages of the tide made it appear that the masonry was too high for the dropping of ramps at any time. As a solution, G–2 officers hit upon a device reminiscent of the storming of castles during the Middle Ages. Scaling ladders were recommended with the suggestion that they be built of aluminum with hooks at one end to be attached to the masonry. Construction was started at Kobe, but the order could be only partially filled before D-day, and wooden ladders were built as substitutes.
It is hardly necessary to point out the importance of estimates as to the numbers and defensive capabilities of the enemy. Yet the G–2 sections on the USS Mount McKinley (AGC-7) were up against a peculiar situation cited in the 1st Marine Division report:
“Our accumulated knowledge of the enemy’s military tactics, prior to our landing at Inch'ŏn on 15 September 1950, consisted almost in its entirety of knowledge about the enemy’s offense. . . . With but few exceptions, UN forces were forced to take a defensive stand and denied the opportunity to study large scale enemy defensive tactics from actual combat. Thus it was that our assault landing was made with relatively little prior knowledge regarding the enemy’s probable reaction to a large-scale offensive of this nature, particularly when it involved the penetration into the very heart of his newly acquired domain”
Photographic coverage showed the Inch'ŏn harbor area to be honey-combed with gun positions and other defensive installations. On the other hand, daily aerial observation indicated that most of them were not occupied. G–2 conclusions during the planning phase often had to be based on such conflicting evidence, even though the penalties of faulty interpretation might be drastic. But after being viewed with due suspicion, signs of negative enemy activity were finally accepted as valid in estimates of light to moderate NKPA resistance.
“Sadly lacking as was information on the objective area,” commented the Division G–2 report, “more so was that on the enemy in the area.”
Early in September, however, the Attack Force and Landing Force concurred in the initial X Corps estimate of 1,500 to 2,500 NKPA troops in the immediate area, consisting largely of newly raised personnel.
Radio reports of first-hand observations in the objective area, though coming too late for initial planning purposes, confirmed some of the G–2 estimates. This dangerous mission was undertaken by Lieutenant Eugene F. Clark, a naval officer on General MacArthur’s JSPOG staff. U.S. and British Marines provided an escort on 1 September when the British destroyer HMS Charity (R-29) brought him from Sasebo to a point along the coast where the South Korean patrol vessel PC-703 Sam Kak San waited to land him at Yŏnghŭng-do, an island about 15 miles southwest of Inch'ŏn.
Clark went ashore with a small arsenal of firearms, grenades and ammunition, as well as 30 cases of C rations and 200 pounds of rice. He quickly made allies of the 400 friendly Korean inhabitants of the island and organized his own private little “army” of about 150 youths from 14 to 18 years old. These “troops” were posted about Yŏnghŭng-do for security, since the near-by island, Taebu-do, was occupied by 400 NKPA soldiers within wading distance at low tide.
The naval officer had no illusions as to what his fate might be in the event of capture. Day and night, he kept a grenade within reach, since he did not intend to be taken alive. When the long expected enemy attack from Taebu-do materialized, he commandeered a “one-lung” South Korean motor sampan and fought it out with the NKPA motor sampan escorting boats filled with soldiers. The enemy began the strange “naval” battle with a few badly aimed rounds from a 37mm tank gun. Clark and his crew of three friendly Koreans finished it with a long burst from a .50 caliber machine gun. After sinking the NKPA motor sampan, he destroyed another boat with 18 soldiers aboard and captured three prisoners for questioning.
One night the intrepid lieutenant rowed a dinghy to the Inch'ŏn sea wall. When the tide went out, he tested the mire by wading in it up to his waist. This experience led to the sending of a radio report, “Inch'ŏn not suitable for landing either troops or vehicles across the mud.”
Korean youths, posing as fishermen, brought intelligence which Clark included in his daily radio messages. One of these spies made an effort to count the guns on Wŏlmi-do and describe the locations. Others took measurements of the Inch'ŏn sea wall and penetrated as far inland as Sŏul to report numbers and positions of NKPA troops.
Clark declined all offers to evacuate him. As the climax of his exploit, he managed to restore the usefulness of the lighthouse on Palmi Island which the enemy had put out of commission. This structure, the former entrance beacon for Inch'ŏn by way of Flying Fish channel, served him as a refuge when he had to leave Yŏnghŭng-do hurriedly just ahead of NKPA troops who landed in force and butchered 50 civilians of both sexes. Clark, who received a Silver Star, stuck it out on Palmi until midnight of 14 September, when he turned on the beacon light to guide the amphibious task force.
 JCS dispatch 93709, 9 Oct 1950.
 EUSAK War Diary, 1 October 1950.
 Message from the U.S. Ambassador to England to Secretary of State, 3 October 1950.
 Personal interview, 25 October 1950.
 General MacArthur’s letter to authors, dated 19 March 1956.
[13A] This Russian predilection for mines is very evident in the Soviet Navy today. Nearly every Soviet
combatant ship—cruiser, destroyer, escort vessel, and submarine—is fitted for minelaying. Russian aircraft can
lay mines as well.
 Notes to authors from Captain N. B. Atkins dated 24 April 1956.
[14A] AMS is hereafter used to designate the small 136-foot wooden-hulled minesweeper to help distinguish it
from the steel-hulled AM.
[14B] Throughout the Korean War, the minesweepers were designated as follows: The destroyer minesweeper
was designated DMS; the steel-hulled fleet minesweeper was designated AM; the wooden-hulled sweeper, AMS
and the converted small boat (LCVP) designated MSB. Throughout this book, these designations will be used.
Subsequently, the designations have been changed.
 Personal interview, October 1955.
[15A] Admiral Struble had been Commander Mine Force Pacific at the end of World War II. He had participated
in 22 amphibious operations and had commanded several. Many of these involved minesweeping.