Chemical mortar battalions were United States Army non-divisional units that were attached to infantry divisions during World War II. They were armed with 4.2-inch (107 mm) chemical mortars. For this reason, they were also called the "Four-deucers".

In 1943, General Mark Clark's Fifth Army established a policy that no infantry division would be committed to combat without a chemical mortar battalion attached. As a result, when infantry units were rotated out of combat, the mortar battalions often stayed in the line and were attached to fresh units. Chemical mortars were in such high demand that often the companies of a battalion would be split up and assigned to different divisions. On the first day that General George S. Patton's Third Army became operational, in the summer of 1944, he issued a standing order to his staff that no infantry division would be committed to combat without a chemical mortar battalion attached, and no infantry regiment would be committed without a mortar company attached.

Mortars came to be acknowledged by U.S. Army commanders and personnel as being one of the most effective means of quickly striking at stationary targets, such as machine gun nests, prepared strongpoints, pillboxes and even artillery positions. Other advantages mortars offered compared to full-sized artillery pieces were their easier transportation, assembly, and disassembly. Thanks to their relatively small size, mortars were able to fire from concealed positions, such as natural escarpments on hillsides, or from woods.

The 4.2 in (107 mm) chemical mortar was developed from the British World War I-era 4 inch (101.6 mm) Stokes mortar. The Stokes mortar could fire twenty shells per minute and had a range of 1,100 yards (1,000 m) and in this way was capable of overwhelming enemy trenches. The American-built M1 4.2 inch (107 mm) mortar, first introduced in 1924, had a rifled barrel and a range of 2,300 yards (2,100 m). By the 1930s, after modifying the bore, improving the two-legged support and the recoil mechanism, and producing barrels made of seamless nickel steel, the M1A1 model was capable of sending shells 2,400 yards (2,200 m)

Chemical mortars were so named because of their original intent of firing poison gas, incendiary and smoke marker shells. Chemical shells were on standby during World War II, to be used in retaliation should the enemy employ chemical weapons first. By 1942, after authorization had been sought and granted to use high explosive shells, the new M2 model was produced with a stronger barrel. Its rate of fire was 40 rounds in the first two minutes, 100 rounds in the first 20 minutes and thereafter a sustained rate of 80 rounds per hour. These variations were caused by the stresses and strains on the barrels and the rest of firing mechanisms that were being imposed by different firing conditions.

Later, the mortar was developed to be capable of firing shells from a mere 565 yards (517 m) at minimum propellant charge, to a range of 7,450 yards (6,810 m). The propellant charges were manufactured as square disks with a hole in the middle, strung together, fitted into stacks, and sewn together into bundles of various thicknesses, often referred to as cheese charges due to their appearance. The rifled barrel gave the mortar remarkable accuracy; fire was often called on targets within fifty yards of friendly positions. The low-velocity shells were totally silent in transit and gave no warning of their powerful explosions (the M2 mortar's M3 high explosive shell contained 3.64 kilograms (8.0 lb) of explosives, placing it midway between the 2.18 kilograms (4.8 lb) of the 105 mm howitzer M2A1's M1 shell and the 6.88 kilograms (15.2 lb) of the 155 mm howitzer M1's M102 shell), which tended to create panic among enemy forces who were unexpectedly subjected to their firepower. The mortar was called the "grass-cutter" by German troops because its high explosive shell exploded and fragmented just a few inches above ground level. The mortars often fired white phosphorus munitions (WP) shells to block enemy observation with smoke; white phosphorus also caused casualties and fires, being especially effective against dug-in troops because the burning particles arced upward and fell directly down into foxholes.

Chemical mortar battalions had their origins in gas regiments, which were first organized in the interwar period. They were redesignated chemical regiments in 1929. The 1st and 2nd Gas Regiments were allotted to the Regular Army. The 1st Gas Regiment had seen service in World War I. It was inactivated on 15 April 1935 and concurrently reorganized entirely with Organized Reserve personnel as a Regular Army Inactive unit. It was inactivated on 9 October 1940 at Philadelphia, Pennsylvania, by transfer of personnel to the 902nd Chemical Regiment. The 2nd Chemical Regiment was constituted in the Regular Army 1 April 1931, assigned to the Zone of the Interior, and allotted to the Fourth Corps Area. Company C, the only active unit, was organized 1 April 1931 at Fort Benning, Georgia, from Company D, 1st Chemical Regiment. The remainder of the regiment was organized in June 1932 with Organized Reserve personnel as a Regular Army Inactive unit at Columbus, Georgia. It was inactivated on 25 August 1940 at Columbus by relief of personnel.

There were two separate Regular Army chemical battalions, one animal-drawn and one motorized. The animal-drawn 1st Chemical Battalion was located in Hawaii due to the mountainous terrain there. Company A, the only active unit, was organized 4 March 1931 at Schofield Barracks from the 2nd Separate Chemical Company. It was disbanded on 12 March 1942. The 2nd Chemical Battalion was organized on 16 April 1935, less Companies B and C, at Edgewood Arsenal, Maryland, and performed duties as the support unit for the Chemical Warfare School. Companies B and C were allotted to the Third Corps Area as inactive units and organized with Organized Reserve personnel.