The Fourth Marines were sent back to Guadalcanal and began training for Operation Iceberg, the invasion of Okinawa.
They were now part of the 6th Marine Division. Besides the 4th and 22d Marines the division’s major elements were the 29th and 15th Marines, the latter being an artillery regiment. The 6th Marine Division with the 1st Marine Division formed the major part of the III Amphibious Corps which itself was a major unit of the Tenth Army, the force that was responsible for the seizure of Okinawa. The overall command of the Tenth Army was given to Lieutenant General Simon B. Buckner, Jr., USA.
The invasion of Okinawa proved to be the last major amphibious assault of the Pacific War. Okinawa’s significance lay in its proximity to the Japanese home islands–only 350 miles from southern Japan.
The day for the invasion of Okinawa was to be April 1st, Easter Sunday that year and was called L Day. The initial mission for the 6th Division was the capture of the Yontan airfield in the central part of the island. The division would drive into the northern parts of the island once Yontan was secure.
The first assault waves hit the beaches at 0837. The 4th Marines, less the 2d Battalion which was in reserve, was among the first units to go ashore. The absence of enemy resistance was an unexpected surprise for most Marines. The advance inland was rapid and significant gains were made the first day. The 2d Battalion was put ashore at noon as a result. Yontan Airfield was swiftly taken by the division on the first day.
According to preliminary plans, but far ahead of schedule, units of the 6th Division then turned north. Northward progress was also rapid with the division reaching the Motobu Peninsula on 7 April. There it encountered its first serious opposition of the campaign. Japanese and Okinawan forces in this mountainous region had emplaced a number of defense obstacles in the path of the advancing Americans.
The job of taking this bastion fell to the 4th Marines and the 3d Battalion, 29th Marines while the 22d Marines and the rest of the 29th Marines were given the job of sealing off the peninsula. The attack began at 0830 on 14 April with a preliminary assault by 2/4 and 3/29 on a 700-foot ridge that was on the rim of Mount Yaetake. A few hours later 1/4 was brought up.
The struggle for the mountain was bitterly contested until 16 April. Marines were forced to seek out and destroy the well-concealed enemy. The Japanese were able to persevere in their resistance in spite of continued infantry attacks that were supported by air and sea bombardment.
Leathernecks of the 6th Division quickly pushed northward once Mount Yaetake was in American hands. Enemy resistance was limited to ineffective harassing tactics. The capture of Motobu Peninsula was completed on 20 April. Organized resistance in northern Okinawa ended a day later and that area was declared secure by Major General Lemuel C. Shepherd, Jr., commander of the 6th Division.
In the south, on the other hand, progress against the enemy had been halted at the Shuri Line. The Army’s XXIV Corps had fought hard to breach Japanese fortifications along this defensive position. General Buckner and his staff decided that a new drive there would necessitate the sending of reinforcements to the zone. The III Amphibious Corps, as a consequence, was directed to redeploy to southern Okinawa and to move into the line to the left of the XXIV Corps.
All major elements of the 6th Division completed the transfer south by 6 May. The Tenth Army, shortly thereafter, launched another effort to achieve a breakthrough in the Shuri Line. The 6th Division in its phase of the offensive was committed to the capture of Naha, the capital. The 4th Marines role in this engagement began on 19 May when it moved out of reserve and relieved the weary 29th Marines. The regiment remained in the thick of the fighting for 10 days, trying to wrest control of the area from the Japanese. The battle at times was so intense that the Marines had to dislodge their adversaries in hand to hand combat. Once the exhausted men of the 4th Marines reached Nahà they were relieved by the 29th Marines.
The 4th’s next assignment was the assault on the Oroku Peninsula on which Naha Airfield was located. An amphibious landing was ordered and the regiment was given the primary responsibility of making it. The 1st and 2d Battalions under a blanket of naval and artillery support fire landed at dawn on 4 June, followed somewhat later by the 3d Battalion. As fate would have it this final assault landing of World War Two by American forces was led by former Raiders in the 4th Marines.
The 29th Marines was put into the line that afternoon to reinforce the regiment. The two regiments slugged it out with the well entrenched enemy for a week and a half. Their progress against the fanatical Japanese was hampered by torrential rains and mud. The enemy had no choice but to surrender or die fighting. As a result, organized resistance quickly ended.
General Shepherd reported to Tenth Army Headquarters on 13 June that the peninsula was indisputably in American hands. Sixth Division units following the conclusion of the battle for the Oroku Peninsula proceeded south and linked up with 1st Marine Division forces for the final engagement on Okinawa. The 4th Marines moved up to the front on the 19th and joined in the attack on the next day. Although some resistance was encountered, the ferocity of the fighting was considerably diminished. For the most part the starch had been taken out of the Japanese will to resist. The effect of this was the termination of all organized resistance on 21 June 1945. The 4th Marines had suffered over 3,000 casualties in 82 days of fighting. The month of July saw the redeployment of the regiment to Guam for training for the expected, but ultimately unnecessary, invasion of Japan.