Battle of Okinawa 65 Years Later — 07 June 1945

07 June 1945

On Okinawa, in the Oroku peninsula, Japanese forces hold attacks by the US 6th Marine Division while the US 1st Marine Division advances southward and isolates the peninsula defenders.

The US 24th Corps is engaged in artillery bombardments.

Okinawa Background — Colonel Unmacht’s 4.2 inch Mortar Gunboats

LVT amphibious tractors move past LCI(M)-809 (center), bound for the Okinawa landing beaches, 1 April 1945.

The redoubtable Colonel Unmacht of Hawaiian flame throwing tank fame was also responsible for another major innovation in off-shore fire support in WW2 — the 4.2 inch Mortar Gunboat.

They were simply LCVP, LSM and LCI landing craft given one to four 4.2 inch (107mm) mortars to provide fire support for landings. The inability of naval gunfire to hit reverse slopes and the short 1,200 yard range of naval 4.5 inch and 5 inch rockets means that the 3,500-4,500 yard range 4.2 inch mortar was ideal to hit the backs of hills and mountains fronting landing beaches.

This is the time line of 4.2 inch gunboat development which supported not only the Central Pacific, but also General Mac Arthur’s SWPA command and the invasion of Sicily!

1942 – Developing doctrine and experimenting
July 1943 – Sicily
Spring 1944 – Saipan, Marianas Group – aborted operation
21 June 1944 – Bougainville, first successful amphibious combat operation
August 1944 – Training in Empress Augusta Bay, Bougainville, Solomon Islands
15 September 1944 – Peleliu, Palau Islands: first LCI(M) combat use
20 October 1944 – Leyte, Philippine Islands
9 January 1945 – Luzon, Philippine Islands
19 February 1945 – Iwo Jima, Bonin Islands, northwest Pacific basin
1 April 1945 – Okinawa, the Ryukyus Islands, northwest Pacific basin

The predominant Mortar gunboats in the Pacific were the LCI(M) followed by the LSM(R) .

This was the performance of the 4.2 inch Mortar Gunboats at Okinawa:

1 April 1945 – Okinawa, the Ryukyus Islands, northwest Pacific basin
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Almost due west and slightly north of Iwo Jima, even closer to the Japanese home island of Kyushu, lies Okinawa. Large scale use of suicide (Kamikaze = Divine Wind) aircraft and torpedo boats were employed against the landing and support forces. Resistance on shore was increasingly determined and suicidal, a harbinger of what to expect in the already scheduled Operations: Coronet against Kyushu, and Olympic against Honshu (the main island).
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CWS-trained Navy crewmen manned a total of 60 LCI(M)s 180 4.2″ mortars in support of Tenth Army in the Ryukyus Campaign. Six days before the main assault of Okinawa , two 6-boat mortar groups (36 4.2″ mortars) supported the diversionary effort against Keramo Retto made by the 77th (New York Statue of Liberty) Infantry Division. On 1 April 1945, the XXIV Army Corps and the III Amphibious Corps successfully carried out the main landings on the western coast of Okinawa. Prior to H-hour on that morning, seven groups of LCI(M)s, each comprised of six boats (7 x 6 x 3 = 126 4.2″ mortars) lined up parallel to the beach behind the assault troops. Each LCI(M) carried 1,000 rounds of HE and 200 rounds of WP. Using Plan Baker, the 42 boats moved through a calm sea at about one knot, their 126 mortars opening up at a point 1,600 yards from shore at a rate of 10 rounds per gun per minute. Firing over the heads of advancing troops, the mortars, in less than an hour, placed about 28,000 rounds on a beach area 1,000 feet deep and 5 « miles wide. The mortar boats themselves received no enemy fire.
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Another group of LCI(M)s supported the 2nd Marine Division’s L-Day feint against the southeast coast of Okinawa. Subsequent 77th Division landings at Ie Shima (where the famous WWII correspondent Ernie Pyle was killed) on 16 April received the support of two groups of mortar boats, while three days later a single group fired for the ruse landing made by the same division in southern Okinawa. From 7 May until 27 June, LCI(M)s, in support of Army and Marine troops, shelled the city of Naha and enemy installations in the vicinity of the capital.
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The amphibious use of the 4.2-inch mortar was one of the major contributions of the CWS to the Pacific war. The mortar boat proved extremely effective for close support just before, during and immediately after amphibious landings. It was then that the assault troops, running the gauntlet of enemy fire while attempting to secure a foothold on the beach, benefited from all the support fire that could be provided. The effectiveness of the mortars in this support is best reflected in the steady increase in the number of mortar boats committed to Pacific assault operations. Only four LCI(M)s saw action in the Palau fighting in September 1944. Seven months later a total of sixty supported Tenth Army operations in the Ryukyus. [Vol. III, pp. 532-33]

For the canceled invasion of Japan, there would be over 100 4.2 inch Mortar Gunboats to bombard the beaches of Kyushu. Not bad for a weapon whose primary purpose was to deliver lethal war gases and which didn’t even have a high explosive shell on Dec 7th 1941!