In the run up to its entrance to World War I, the United States Army found itself lacking in heavy artillery. It developed a crash program to develop heavy artillery pieces and found assistance from the French Army. At the end of the war, the US Army then developed plans for improvements on the heavy artillery at their disposal. While they had developed prototypes for use in the 1920s and 1930s, lack of funds prevented them from proceeding with development. It was not until 1938 that the US Army was finally able to produce a new heavy artillery piece for use, designating it in two variants that are known as the 155mm Gun M1 and 155mm Gun M2. Already after the first 20 or so had rolled off the production lines, the army implemented changes to improve its effectiveness. Eventually it developed the nickname of the “Long Tom”.

The huge gun was normally transported to the front by heavy trucks and specialized prime movers but could be built on a tracked chassis and therefore be a self-artillery piece. It could fire a variety of shells up to 23,500 meters. Furthermore, the wheels on the towed artillery piece could lift up from the ground and the gun mounted on a firing platform, which improved stability when it fired its shells.

The 155 M1s and M2s first saw action in North Africa and eventually saw action by the US Army in both the European Theater of Operations and Pacific Theater of Operations. They operated at corps command level and proved popular with the troops. In fact, they proved at times too popular as army units came to rely on them extensively such as in the Italian campaign where Allied forces had to literally batter their way through extremely though German defenses. This resulted in concern about maintaining the large gun barrel to ensure it would work effectively. In addition, the US gave several of these weapons to Britain and France under the “Lend-Lease” Program.

After the war, the US Army re-designated the gun as the M59 and soon lent it to other countries such as South Korea, which would use this type of weapon and other similar ones five years later during the Korean War. Despite official denials and lack of verifiable information, there had been reports that the Japanese Self Defense Force has gotten access to such guns as well.

When an alien race called the Mysterians began to invade the Earth and already made a secret beachhead near Mt. Fuji in Japan, the Japanese government took the lead in trying to negotiate in terms of discovering their intentions. After finding out the Mysterians were hostile, the JSDF moved into action. Alongside Honest John Missile launchers, the 155mm M2s fired in support of M4A3E8 Sherman tanks as they tried to destroy the huge dome-like structure that served as the headquarters of the Mysterians. The bombardment by conventional weapons had no effect, as the armor that surrounded the large dome was not of this Earth and eventually the 155mm M2s suffered heavy losses as the Mysterians retaliated with a radioactive ray from the dome, which literally melted the guns and the other weapons of the JSDF into radioactive goo.

When a very ancient civilization called Mu rose up from the ocean's of the Earth to claim the rest as their new colony, the 155mm M2s were seen moving into action alongside other JSDF weapons. Fortunately, they would not go into action as a powerful secret submersible battleship called Atragon took on the Mu Empire and destroyed it in its very lair.

A new threat from space emerged in the form of Dogora, a mysterious jellyfish-like creature that consumed large amounts of carbon based material particularly crystals and coal. In an attempt to deal with this airborne threat, the 155mm M2s were pressed into service alongside other guns in a surprising role, that of anti-aircraft weapons. At first, it seemed that after a long bombardment during a night engagement over one of the main northeastern cities in Kyushu, they managed to drive the creature away but instead all they did were to blast pieces of the creature's skin off from the main body, which were capable of forming new ones. Fortunately, though, the JSDF along with assistance of scientists from all over the world managed to find a weakness in that the cells of Dogora were vulnerable to the toxin from wasps. Immediately production got underway and soon they had enough of the toxin to meet Dogora's next attack. The next time the guns went in, they mainly participated in a holding action while the toxin tanks and other weapons based on the manufactured wasp toxin moved into position to kill the alien creature.