On February 19, 1945, two United States Marine Corps divisions landed on the sulfurous shores of Iwo Jima. As was customary, prior to the actual landings, the small 7.5-square-mile island received a heavy preparatory bombardment from the 16-inch and 14-inch guns of the Navy’s battleships and the smaller guns of the cruisers and destroyers offshore. When the actual assault landings began, waves of landing craft moved toward Green, Red, Yellow and Blue beaches. Accompanying the landing craft were 18 LCI(M)s (landing craft, infantry [mortar]). The force was divided into three groups of six craft each. All the LCI(M)s were equipped with 4.2-inch mortars to provide close fire support to the Marines as they reached the shore.

At first, each of the LCI(M) groups took up positions offshore to bombard targets on Mount Suribachi. In each of the three groups, one of six LCI(M)s would sit in the center of the group and serve as a reference point while the five remaining craft would circle around and fire as their bows came around to point at the target. Working with speed and professionalism, the crews of the LCI(M)s had delivered 3,240 rounds at Japanese positions before the first Marines stepped into the surf.

The groups then reorganized themselves, and two hours into the landing (Hplus-2), two of the groups joined the sixth assault wave as it approached the shore. When they were 2,000 yards from the beach, they began to fire at a set range of 3,200 yards, continuing this as they moved steadily closer to the shore. At a range of 1,000 yards the two groups stopped and concentrated their fire on a line 1,800 yards from the beach. This supporting fire was kept up until H-plus-60, when both LCI(M) units retired. After the initial assault, a total 30 LCI(M)s provided fire support during the battle until the last boats were recalled on March 3. Marines ashore appreciated the massed fire provided by the mortar boat units, and Rear Adm. Harry W. Hill said that the use of the LCI(M) was “one of the outstanding features of the operation.”

The concept of arming landing craft with mortars came about as military planners began to consider the many complexities of amphibious operations. One of the most challenging conditions facing troops carrying out an assault landing was the lack of direct fire support after the prelanding naval bombardment had been lifted but before the attacking troops could get ashore and deploy their own artillery and mortars.

Given the irregularities of naval gunfire, it was impossible to maintain offshore fire all the way into the beach. Errant rounds landing short or long of the target in a line extending from the firing ship (known in artillery terminology as “Range Probable Error”) rather than to the left or right of the target (known as “Deflection Probable Error”) could lead to disaster. To prevent such friendly fire accidents, naval bombardments had to be lifted soon enough to prevent short rounds from hitting the assault waves. LCI(M)s could provide accurate close support until the troops could get their own artillery ashore and into action.

One suggestion before the LCI(M) concept was developed called for tanks and artillery pieces taking part in the assault to fire directly from their landing craft’s open deck. Given the distractions of those whose ultimate objective was the beach and the Navy crews preoccupied with getting their loads ashore, such an idea had obvious disadvantages. Better to mount a heavy mortar in a landing craft whose sole purpose was to support the troops going ashore.

Development of such special purpose craft began in mid-1942. The U.S. Army Chemical Warfare Service had created the M2 4.2-inch (107mm) mortar during the 1920s and ’30s. With memories of World War I still lingering in the minds of many, the mortar’s original purpose was to deliver gas and white phosphorus smoke rounds. The mortar’s use as a close-support weapon using high-explosive shells was sidelined. It took the demands of World War II for the War Department to approve the use of high-explosive munitions by chemical mortar units.

For use in support of amphibious assaults, the 4.2-inch mortar offered a number of advantages. As with mortars in general, it had a high rate of fire—up to 20 rounds per minute—and could launch its munitions at a high angle, enabling the weapon to fire at targets hidden in defilade behind hills or trees. With its relatively large caliber, the M2 had the firepower equivalent of a 105mm howitzer. Although at 4,000 yards the mortar had less than half the range of the 105mm howitzer, its rounds could be designed to pack more explosive than those of the howitzer. Another advantage to its use on landing craft was the mortar’s relative lack of recoil—more tubes could be placed in a small craft. If properly secured on landing craft, 4.2 mortars could fire a large number of explosive shells quickly to support an assault landing, as well as being able to lay smoke screens when winds were favorable.

Development of the necessary doctrine for the use of the new mortar-equipped craft began at the Amphibious Training Center at Camp Edwards, Mass., and at Camp Carabelle, Fla. Further work took place at the Assault Training Center in England. By the time of the July 1943 Sicily invasion, six landing craft had been equipped with mortars, though none of them were used during those landings. In fact, chemical-mortar-equipped landing craft never saw active service in any landings in the European theater.

It was a far different story in the Pacific. At the Hawaiian island of Oahu, in a rare cooperative effort between the Army and the Navy, three LCTs (landing craft, tank) were fitted with 4.2-inch mortars. Tests were conducted to find a shipboard mounting for the weapon that could withstand the shock of firing without excessive parts breakage. The final version was a wooden box, filled with sand and sawdust and reinforced with steel. The mortar baseplate was then attached to a wooden “sub base,” which was then set on top of the sand-sawdust mixture.

After successful testing at Oahu, three LCTs with mortars were earmarked for the invasion of Saipan. Misfortune plagued the operation, however, and the LCTs did not arrive in time for the June 1944 invasion. Each LCT was to be carried on a larger LST (landing ship, tank). During the invasion rehearsal off the Hawaiian island of Kahoolawe, two of the LCTs were washed overboard in heavy seas. The third was lost when the LST carrying it was destroyed in an explosion at Pearl Harbor.

This inauspicious beginning was not enough to end the mortar boat project, though. The Navy persevered, this time using LCIs. Designated LCI(M), each would mount three 4.2-inch mortars. Two were mounted amidships to port and starboard, and one on the forward centerline. Magazines were installed in the forward troop compartments. These magazines normally held 1,200 rounds of ammunition. Finally, steel plating was used to reinforce the decks under the mortars. The weapons were manned by soldiers from the 88th Chemical Mortar Battalion.

The LCI(M)s first saw action in the Palau islands in mid-September 1944, the first rounds being fired on September 15, when the mortar craft carried out two missions in support of assault troops. In the first, LCI(M)s moved slowly toward shore, firing only one round every two minutes from each mortar, at ranges of 3,000 to 1,300 yards offshore. This fire covered the targeted beach 200 to 300 yards inland. After ceasing fire to clear the way for friendly airstrikes, the mortar boats moved closer to shore and resumed their strikes at suspected enemy positions. The second mission began an hour later on the same positions, firing two rounds per minute from each mortar. During these engagements, the mortarmen learned that although the landing craft’s movement and drift in the water made it more difficult to fire accurately, they were able to conduct effective fire missions.

Two days later, the LCI(M)s supported the landings of the 81st Infantry Division on Angaur Island; using rapid-fire techniques, they fired more than 2,300 rounds in just 17 minutes. Three days later, the mortarmen were called upon to support infantry pinned down by enemy fire on the island’s northwest end. The target area went inland about 900 yards from the shoreline with a varying width of 500 to 850 yards. Though minefields kept the boats 1,500 yards offshore, the four vessels fired 830 rounds into the relatively small target area. This fire proved instrumental in allowing the infantry to subsequently resume its advance.

As the tempo of operations in the Pacific increased, more and more landing craft were converted to mortar carriers. During the invasion of the Philippines in October 1944, two groups of four LCI(M)s took part in the landings on Leyte. There, they followed much the same pattern, moving closely with the assault waves, firing preparation bombardments during the movement toward shore and then assuming fixed positions and providing supporting fire. While assisting infantry near the Daguitan River, the LCI(M)s served as supporting artillery and answered urgent calls for fire against Japanese forces on the southern bank of the river near an important bridge. The Americans’ accurate mortar fire prevented Japanese tanks from crossing the bridge and attacking the vulnerable infantry. At the conclusion of the invasion, both mortar boat groups received written commendations from the flotillas they supported as well as from two amphibious group commanders.

The next task for the boats came during the Luzon landings in January 1945. This time there were three mortar groups of six, four and three boats each, supporting General Douglas MacArthur’s invasion. The group of four vessels had been hastily created by converting four LCIs that had previously been used as ammunition carriers, while the three-boat group was manned by Marines trained by the Army’s 98th Chemical Mortar Battalion. Although from different services, at both Luzon and Dagupan landings the groups followed the doctrine that had evolved over time and moved inland alongside the initial assault waves, firing as they went, before taking up positions close to shore. At the end of the operation, Vice Adm. Thomas Kinkaid stated that the LCI(M)s neutralized beach defenses more effectively than carrier aircraft.

The proven effectiveness of the mortar craft led to the construction of even more of them. The new boats were crewed exclusively by Navy personnel. Thirty were available for the attack on Iwo Jima in February 1945. Afterward, all of them performed harassment and interdiction fire, doing what they could to slow down and interfere with enemy movement. These missions were conducted at night. Used for the first time in their originally intended role, the 4.2s fired large numbers of white phosphorus shells at the defenders.

In response, the Japanese began concentrating much of their own considerable artillery fire at the LCI(M)s. To protect the vital mortar boats and their crews, on the evening of February 23, the Navy sent the destroyer Shannon close inshore. The mortar boat groups were gradually withdrawn as the battle wound down, but by the end of the invasion they had fired an impressive 60,000 rounds, including nearly 12,000 rounds of white phosphorus.

The last action for the LCI(M)s came during the invasion of Okinawa. By then 60 mortar boats, crewed by sailors trained by the Army, were in service for the attack. Twelve boats supported the 77th Infantry Division’s diversionary attack at Kerama Retto on March 26, while 42 took part in the main assault on April 1.

Moving slowly toward the beach, each of the three mortars aboard the LCI(M)s began firing at 10 rounds per minute beginning at a range of 1,600 yards offshore. The target area was a section of beach 51⁄2 miles wide and 1,000 feet deep. The mortar craft hit it with more than 28,000 rounds in less than an hour. Unlike at Iwo Jima, there was no fire from ashore. Afterward, the mortar boats supported the invasion troops until the end of June.

The successful use of the 4.2-inch mortar aboard landing craft is best proven by their proliferation. During their relatively short term of service, from September 1944 to June 1945, the number of LCI(M)s increased dramatically. The initial group of four boats grew to 60 by war’s end. They provided fire support during the most desperate phase of an amphibious assault, when the landing forces were trying to establish a secure beachhead. And they are yet another example of the the American military’s ability to adapt to the requirements of a global war and take advantage of innovations and improvisations that made victory possible.


Originally published in the February 2006 issue of World War II. To subscribe, click here.