The Soviet Union occupied the northern half of Korea after World War II ended in 1945.On June 25, 1950, the North Korean army attempted to end that split by invading across the 38th parallel dividing line between North and South:
Southern Korea was controlled by the United States.
The peninsula was formally split in two on 9 September 1948.
At approximately 4 a.m. (Korean Standard Time) on a rainy Sunday morning Democratic People's Republic of Korea Army (DPRK - North Korea) artillery and mortars open fire on Republic of Korea (ROK - South Korea) Army positions south of the 38th Parallel, the line then serving as the border between the two countries. The opening barrage is followed shortly by tank/infantry attacks at all points along the Parallel. At 11 a.m. North Korea announced a formal declaration of war and what is now known as "The Korean War" officially began.On June 27, the UN acted:
During the late evening, the U.N. Security council passes a resolution calling for member nations to give military aid to South Korea.As mobilization of forces begins, limited American and UN forces are deployed into South Korea. The North Korean army continues its advance, pushing the UN forces into a relatively small area on the southeast coast around the port of Pusan. This area becomes known as the "Pusan Perimeter":
On August 1 the Eighth Army issued an operational directive to all UN ground forces in Korea for their planned withdrawal east of the Naktong River. UN units would then establish main defensive positions behind what was to be called the Pusan Perimeter. The intent was to draw the line on retreating and hold off the Korean People's Army while the U.S. Army could build up its forces and wage a counteroffensive.General MacArthur was in command of the UN forces in Korea, and was looking for a way to relieve the pressure on the defenders of the Pusan Perimeter and to go on the offensive. He came up with a stunning plan - an amphibious invasion that would serve to cut the North Korean army supply lines and open up a new front in the war. Even more stunning was his choice for the location of the invasion- the port of Inchon, up near the South Korean capital of Seoul. Inchon is port noted for its extreme tidal ranges and its difficult approach channels. It should be noted that General MacArthur's mind turned to an amphibious operation quite early in the war- according to this, within days of the North Korean invasion:
The Pusan Perimeter assumed by U.S. and ROK forces on August 4 involved a rectangular area about 100 miles (160 km) from north to south and 50 miles (80 km) from east to west. The Naktong River formed the western boundary except for the southernmost 15 miles (24 km) where the Naktong turned eastward after its confluence with the Nam River. The ocean formed the eastern and southern boundaries, while the northern boundary was an irregular line that ran through the mountains from above Waegwan to Yongdok.
During the first week of July, with the Korean War little more than a week old, General MacArthur told his chief of staff, General Almond, to begin considering plans for an amphibious operation designed to strike the enemy center of communications at Seoul, and to study the location for a landing to accomplish this. At a Far East Command headquarters meeting on 4 July, attended by Army, Navy, and Air Force representatives, Generals MacArthur and Almond discussed the idea of an amphibious landing in the enemy's rear and proposed that the 1st Cavalry Division be used for that purpose. Col. Edward H. Forney of the Marine Corps, an expert on amphibious operations, was selected to work with the 1st Cavalry Division on plans for the operation. Assembling a landing force was a challenge as was getting the Joint Chief on board with the idea of Inchon as the target:
The early plan for the amphibious operation received the code name BLUEHEARTS and called for driving the North Koreans back across the 38th Parallel. The approximate date proposed for it was 22 July, but the operation was abandoned by 10 July because of the inability of the U.S. and ROK forces in Korea to halt the southward drive of the enemy. 
Meanwhile the planning for an amphibious operation went ahead in the Far East Command despite the cancellation of BLUEHEARTS. These plans were undertaken by the Joint Strategic Plans and Operations Group (JSPOG), Far East Command, which General Wright headed in addition to his duties as Assistant Chief of Staff, G-3. One of Wright's deputies, Col. Donald H. Galloway, was directly in charge of JSPOG. This unusually able group of planners developed various plans in considerable detail for amphibious operations in Korea.
On 23 July, General Wright upon MacArthur's instructions circulated to the GHQ staff sections the outline of Operation CHROMITE. CHROMITE called for an amphibious operation in September and postulated three plans: (1) Plan 100-B, landing at Inch'on on the west coast; (2) Plan 100-C, landing at Kunsan on the west coast; (3) Plan 100-D, landing near Chumunjin-up on the east coast. Plan 100-B, calling for a landing at Inch'on with a simultaneous attack by Eighth Army, was favored. 
This same day, 23 July, General MacArthur informed the Department of the Army that he had scheduled for mid-September an amphibious landing of the 5th Marines and the 2d Infantry Division behind the enemy's lines in co-ordination with an attack by Eighth Army.
On 12 August, MacArthur issued CINCFE Operation Plan 100-B and specifically named the Inch'on-Seoul area as the target that a special invasion force would seize by amphibious assault. (citations omitted)
All through July and August 1950 the Joint Chiefs of Staff gave implied or expressed approval of MacArthur's proposal for an amphibious landing behind the enemy's battle lines. But while it was known that MacArthur favored Inch'on as the landing site, the Joint Chiefs had never committed themselves to it. From the beginning, there had been some opposition to and many reservations about the Inch'on proposal on the part of General Collins, U.S. Army Chief of Staff; the Navy; and the Marine Corps. The FEC senior planning and staff officers-such as Generals Almond and Hickey, Chief of Staff and Deputy Chief of Staff; General Wright, the G-3 and head of JSPOG; and Brig. Gen. George L. Eberle, the G-4-supported the plan. The Navy almost did, but it seems the net result of Navy concerns was a weasel-worded message to MacArthur which authorized him to hang himself if he so chose, but...
The Navy's opposition to the Inch'on site centered largely on the difficult tidal onditions there, and since this opposition continued, the Joint Chiefs of Staff decided to send two of its members to Tokyo to discuss the matter with MacArthur and his staff. A decision had to be reached. On 20 July General Collins and Admiral Forrest P. Sherman, Chief of Naval Operations, left Washington for their conference with MacArthur. Upon arrival in Japan, Collins and Sherman engaged in private conversations with MacArthur and key members of his staff, including senior naval officers in the Far East. Then, on the afternoon of 23 July, a full briefing on the subject was scheduled in General MacArthur's conference room in the Dai Ichi Building.
After a short introduction by General MacArthur, General Wright briefed the group on the basic plan. Admiral Doyle then presented the naval considerations. His general tone was pessimistic, and he concluded with the remark, "The operation is not impossible, but I do not recommend it." The naval part of the briefings lasted more than an hour.
During the naval presentation MacArthur, who had heard the main arguments many times before, sat quietly smoking his pipe, asking only an occasional question. When the presentation ended, MacArthur began to speak. He talked as though delivering a soliloquy for forty-five minutes, dwelling in a conversational tone on the reasons why the landing should be made at Inch'on. He said that the enemy had neglected his rear and was dangling on a thin logistical rope that could be quickly cut in the Seoul area, that the enemy had committed practically all his forces against Eighth Army in the south and had no trained reserves and little power of recuperation. MacArthur stressed the strategical, political, and psychological reasons for the landing at Inch'on and the quick capture of Seoul, the capital of South Korea. He said it would hold the imagination of Asia and win support for the United Nations. Inch'on, he said, pointing to the big map behind him, would be the anvil on which the hammer of Walker's Eighth Army from the south would crush the North Koreans.
General MacArthur then turned to a consideration of a landing at Kunsan, 100 air miles below Inch'on, which General Collins and Admiral Sherman had favored. MacArthur said the idea was good but the location wrong. He did not think a landing there would result in severing the North Korean supply lines and destroying the North Korean Army. He returned to his emphasis on Inch on, saying that the amphibious landing was tactically the most powerful military device available to the United Nations Command and that to employ it properly meant to strike deep and hard into enemy-held territory. He dwelt on the bitter Korean winter campaign that would become necessary if Inch'on was not undertaken. He said the North Koreans considered a landing at Inch'on impossible because of the very great difficulties involved and, because of this, the landing force would achieve surprise. He touched on his operations in the Pacific in World War II and eulogized the Navy for its part in them. He concluded his long talk by declaring unequivocally for Inch'on and saying, "The Navy has never turned me down yet, and I know it will not now."
On 28 August the Joint Chiefs sent a message to MacArthur which seemingly concurred in the Inch'on plans yet attached conditions. Their message said in part: "We concur in making preparations for and executing a turning movement by amphibious forces on the west coast of Korea, either at Inch'on in the event the enemy defenses in the vicinity of Inch'on prove ineffective, or at a favorable beach south of Inch'on if one can be located. We further concur in preparations, if desired by CINCFE, for an envelopment by amphibious forces in the vicinity of Kunsan. We understand that alternative plans are being prepared in order to best exploit the situation as it develops.Knowing what he wanted, MacArthur ignored the message and continued planning for Inchon. Again the Joint Chiefs urged rethinking and MacArthur replied:
There is no question in my mind as to the feasibility of the operation and I regard its chance of success as excellent. I go further and believe that it represents the only hope of wresting the initiative from the enemy and thereby presenting an opportunity for a decisive blow. To do otherwise is to commit us to a war of indefinite duration, of gradual attrition, and of doubtful results.... There is no slightest possibility . . . of our force being ejected from the Pusan beachhead. The envelopment from the north will instantly relieve the pressure on the south perimeter and, indeed, is the only way that this can be accomplished.... The success of the enveloping movement from the north does not depend upon the rapid juncture of the X Corps and the Eighth Army. The seizure of the heart of the enemy distributing system in the Seoul area will completely dislocate the logistical supply of his forces now operating in South Korea and therefore will ultimately result in their disintegration. This, indeed, is the primary purpose of the movement. Caught between our northern and southern forces, both of which are completely self-sustaining because of our absolute air and naval supremacy, the enemy cannot fail to be ultimately shattered through disruption of his logistical support and our combined combat activities.... For the reasons stated, there are no material changes under contemplation in the operation as planned and reported to you. The embarkation of the troops and the preliminary air and naval preparations are proceeding according to schedule.On the Navy side, planning went forward:
The next day the Joint Chiefs, referring to this message, replied tersely to MacArthur, "We approve your plan and President has been so informed."  It appears that in Secretary of Defense Johnson, MacArthur had in Washington a powerful ally during the Inch'on landing controversy, for Johnson supported the Far East commander.  Thus on 8 September Washington time and 9 September Tokyo time the debate on the projected Inch'on landing ended.In making ready its part of the operation, the Commander, NAVFE outlined the tasks the Navy would have to perform. These included the following: maintain a naval blockade of the west coast of Korea south of latitude 39° 35' north; conduct pre-D-day naval operations as the situation might require; on D-day seize by amphibious assault, occupy, and defend a beachhead in the Inch'on area; transport, land, and support follow-up and strategic reserve troops, if directed, to the Inch'on area; and provide cover and support as required. Joint Task Force Seven was formed to accomplish these objectives with Admiral Struble, Commander, Seventh Fleet, as the task force commander. On 25 August, Admiral Struble left his flagship, USS Rochester, at Sasebo and proceeded by air to Tokyo to direct final planning. Final preparations included some last minute checking on conditions near Inchon:
On 3 September, Admiral Struble issued JTF 7 Operational Plan 9-50. Marine aircraft from two escort carriers, naval aircraft from the U.S. carrier Boxer, and British aircraft from a light British carrier would provide as much support aircraft as could be concentrated in and over the landing area, and would be controlled from the amphibious force flagship (AGC) Mt. McKinley. An arc extending inland thirty miles from the landing site described the task force objective area.
More than 230 ships were assigned to the operation. Surface vessels of JTF 7 were not to operate within twelve miles of Soviet or Chinese territory nor aircraft within twenty miles of such territory. 
MacArthur had selected Inch'on as the landing site for one paramount reason: it was the port for the capital city of Seoul, eighteen miles inland, and was the closest possible landing area to that city and the hub of communications centering there.
Inch'on is situated on the estuary of the Yom-ha River and possesses a protected, ice-free port with a tidal basin. The shore line there is a low-lying, partially submerged coastal plain subject to very high tides. There are no beaches in the landing area-only wide mud flats at low tide and stone walls at high tide. Because of the mud flats, the landing force would have to use the harbor and wharfage facilities in the port area. The main approach by sea is from the south through two channels 50 miles long and only 6 to 10 fathoms deep (36-60 feet). Flying Fish Channel is the channel ordinarily used by large ships. It is narrow and twisting.
The Inch'on harbor divides into an outer and an inner one, the latter separated from the former by a long breakwater and the islands of Wolmi and Sowolmi which join by a causeway. The greater part of the inner harbor becomes a mud flat at low tide leaving only a narrow dredged channel of about ~13 feet in depth. The only dock facilities for deep draft vessels were in the tidal basin, which was 1,700 feet long, 750 feet wide, and had an average depth of 40 feet, but at mean low tide held only feet of water. 
Inch'on promised to be a unique amphibious operation-certainly one very difficult to conduct because of natural conditions. Tides in the restricted waters of the channel and the harbor have a maximum range of more than 31 feet. A few instances of an extreme 33-foot tide have been reported. Some of the World War II landing craft that were to be used in making the landing required 23 feet of tide to clear the mud flats, and the LST's (Landing Ship, Tank) required 29 feet of tide-a favorable condition that prevailed only once a month over a period of three or four days. The narrow, shallow channel necessitated a daylight approach for the larger ships. Accordingly, it was necessary to schedule the main landings for the late afternoon high tide. A night approach, however, by a battalion-sized attack group was to be made for the purpose of seizing Wolmi-do during the early morning high tide, a necessary preliminary, the planners thought, to the main landing at Inch'on itself. (Note by E1: The nearby photo showing a landing craft (medium) perched on a ship wreck in the Inchon area gives an idea of the tidal range and the hazards faced by the landing force)
Low seas at Inch'on are most frequent from May through August, high seas from October through March. Although September is a period of transition, it was considered suitable for landing operations. MacArthur and his planners had selected 15 September for D-day because there would then be a high tide giving maximum water depth over the Inch'on mud flats. Tidal range for 15 September reached 31.2 feet at high and minus .5 feet at low water. Only on this day did the tide reach this extreme range. No other date after this would permit landing until 27 September when a high tide would reach 27 feet. On 11-13 October there would be a tide of 30 feet. Morning high tide on 15 September came at 0659, forty-five minutes after sunrise; evening high tide came at 1919, twenty-seven minutes after sunset. The Navy set 23 feet of tide as the critical point needed for landing craft to clear the mud flat and reach the landing sites. 
Another consideration was the sea walls that fronted the Inch'on landing sites. Built to turn back unusually high tides, they were 16 feet in height above the mud flats. They presented a scaling problem except at extreme high tide. Since the landing would be made somewhat short of extreme high tide in order to use the last hour or two of daylight, ladders would be needed. Some aluminum scaling ladders were made in Kobe and there were others of wood. Grappling hooks, lines, and cargo nets were readied for use in holding the boats against the sea wall.
At the end of August the ports of Kobe, Sasebo, and Yokohama in Japan and Pusan in Korea had become centers of intense activity as preparations for mounting the invasion force entered the final stage. The 1st Marine Division, less the 5th Marines, was to outload at Kobe, the 5th Marines at Pusan, and the 7th Infantry Division at Yokohama. Most of the escorting vessels, the Gunfire Support Group, and the command ships assembled at Sasebo.
The ships to carry the troops, equipment, and supplies began arriving at the predesignated loading points during the last days of August. In order to reach Inch'on by morning of 15 September, the LST's had to leave Kobe on 1o September and the transports (AP's) and cargo ships (AK's) on 12 September. Only the assault elements were combat-loaded. Japanese crews manned thirty-seven of the forty-seven LST's in the Marine convoy.As a final means of checking on conditions in Inch'on harbor, the Navy on 31 August sent Lt. Eugene F. Clark to Yonghung-do, an island at the mouth of the ship channel ten sea miles from Inch'on. There, Clark used friendly natives to gather the information needed. He sent them on several trips to Inch'on to measure water depths, check on the mud flats, and to observe enemy strength and fortifications. He transmitted their reports by radio to friendly vessels in Korean waters. Clark was still in the outer harbor when the invasion fleet entered it.The photo shows Lt. (later Commander) Clark and some of his men. Commander Clark's book about his adventures, The Secrets of Inchon, is interesting reading, as is this (pdf).
The invasion began with pre-invasion bombardment:The Inchon assault formally began with an air and gunfire bombardment of Wolmi-Do island on 13 September 1950. Six U.S. Navy destroyers led off that morning by steaming up the channel on the flooding tide, the better to avoid any waiting mines. Overhead, carrier planes bombed and rocketed Wolmi-Do, whose artillery batteries threatened the planned landing beaches. Further inland, other planes had been working for several days to isolate the Inchon area from enemy reinforcement.See also here for a destroyer eye view at the
The destroyers anchored off Wolmi-Do shortly after noon and began firing pointblank at targets there and along the Inchon waterfront. Their bombardment continued for about an hour, followed by heavier gunfire from cruisers and more air attacks. North Korean return fire damaged three of the destroyers, killing an officer on USS Lyman K. Swenson (DD-729). Wolmi-Do and other nearby targets were hit again the following day, with good results, and again just before landings began on the morning of 15 September.
DeHaven Sailors Association website.
The landings went forward and the invasion forces worked their way inland. At the same time, the forces in the Pusan Perimeter began a counterattack, pushing the North Korean army to move north, eventually to be caught between the forces brought in through Inchon and those moving out of Pusan.
While valor was the order of the day, one photograph speaks volumes:
Caption:First Lieutenant Baldomero Lopez, USMC, leads the 3rd Platoon, Company A, 1st Battalion, 5th Marines over the seawall on the northern side of Red Beach, as the second assault wave lands, 15 September 1950. Wooden scaling ladders are in use to facilitate disembarkation from the LCVP that brought these men to the shore.
Lt. Lopez was killed in action within a few minutes, while assaulting a North Korean bunker.
Lt. Lopez was awarded the Medal of Honor. Today we honor Lt. Lopez's memory with a Military Sealift Command prepositioning ship named, MV 1st Lt. Baldomero Lopez.
A salute to all those brave men of Inchon!
UPDATE: Added map of Pusan Perimeter from here