ligence Agency–military intelligence reconnaissance, codenamed Trudy Jackson, placed a team of guerrillas in Inchon. The group, led by Navy Lieutenant Eugene Clark, landed at Yonghung-do, an island in the mouth of the harbor. From there, they relayed intelligence back to U.S. forces.
With the help of locals, the guerrillas gathered information about tides, mudflats, seawalls, and enemy fortifications. The mission's most important contribution was the restarting of a lighthouse on Palmi-do. When the North Koreans discovered that the allied agents had entered the peninsula, they sent an attack craft with 16 infantrymen. Eugene Clark mounted a machine gun on a sampan and sank the attack boat. In response, the North Koreans killed up to 50 civilians for helping Clark.
A series of drills and tests were conducted elsewhere on the coast of Korea, where conditions were similar to Inchon, before the actual invasion. These drills were used to perfect the timing and performance of the landing craft.
As the landing groups neared, cruisers and destroyers from several UN navies shelled Wolmi-do and checked for mines in Flying Fish Channel. The first Canadian forces entered the Korean War when HMCS Cayuga, HMCS Athabaskan, and HMCS Sioux bombarded the coast. The Fast Carrier Force flew fighter cover, interdiction, and ground attack missions. Destroyer Squadron Nine, headed by the USS Mansfield, sailed up Eastern Channel and into Inchon Harbor, where it fired upon enemy gun emplacements. The attacks tipped off the North Koreans that a landing might be imminent. The North Korean officer at Wolmi-do assured his superiors that he would throw the enemy back into the sea.
The flotilla of ships that landed during the battle was commanded by Arthur Dewey Struble, an expert in amphibious warfare. Struble had participated in amphibious operations during World War II, including the Battle of Leyte and the Battle of Normandy.
At 6:30 am on September 15, 1950, the lead elements of U.S. X Corps hit "Green Beach" on the northern side of Wolmi-Do Island. The landing force consisted of the 3rd Battalion 5th Marines and nine M26 Pershing tanks from the 1st Tank Battalion. One tank was equipped with a flamethrower (flame tank) and two others had bulldozer blades. The battle group landed in LSTs designed and built during World War II. The entire island was captured by noon at the cost of just 14 casualties. North Korean casualties included over 200 killed and 136 captured, primarily from the 918th Artillery Regiment and the 226th Independent Marine Regiment. The forces on Green Beach had to wait until 7:50 p.m. for the tide to rise, allowing another group to land. During this time, extensive shelling and bombing, along with anti-tank mines placed on the only bridge, kept the North Koreans from launching a significant counterattack. The second wave came ashore at "Red Beach" and "Blue Beach."
The North Korean army had not been expecting an invasion at Inchon. After the storming of Green Beach, the NKPA assumed (probably because of deliberate misinformation by American counter-intelligence) that the main invasion would happen at Kunsan. As a result, only a small force was diverted to Inchon. Even those forces were too late, and they arrived after the UN forces had taken Blue and Red Beaches. The troops already stationed at Inchon had been weakened by Clark's guerrillas, and napalm bombing runs had destroyed key ammunition dumps. In total, 261 ships took part.
The Red Beach forces, made up of the Regimental Combat Team 5, used ladders to scale the sea walls. After neutralizing North Korean defenses, they opened the causeway to Wolmi-Do, allowing the tanks from Green Beach to enter the battle. Red Beach forces suffered eight dead and 28 wounded.
Under the command of Colonel Lewis "Chesty" Puller, the 1st Marine Regiment landing at Blue Beach was significantly south of the other two beaches and reached shore last. As they approached the coast, the combined fire from several NKPA gun emplacements sank one LST. Destroyer fire and bombing runs silenced the North 