A mortar, in armament, is a complete projectile-firing weapon, rifled or smooth bore, characterized by a short barrel, low velocity, short range, and high angle of fire. An infantry mortar is a muzzle-loaded weapon, usually having a smooth bore whose diameter is less than 105 mm. An artillery mortar is a similar weapon whose bore diameter is 105 mm. or larger. Mortars are used to reach nearby targets that are protected or concealed by intervening hills or other short-range barriers.
The earliest weapons using gunpowder, developed from the ballista, might be classified as mortars; from these early mortar-type weapons, guns and howitzers were later developed. Mortars were used to a limited extent in the early period for siege purposes. Their use was revived in the latter part of the 19th century for coast defense. Ships were built with heavy side armor, but their relatively thin deck armor offered an attractive mortar target. A considerable number of 12-inch mortars were installed in harbor defenses.
In World War I, trench warfare resulted in the development of infantry and artillery mortars. At the beginning of the war the Germans used Minenwerfer designed and built in 1911. Both the French and British rapidly developed infantry and artillery mortars to meet the conditions imposed by trench warfare. Calibers ranged from 2 inches to 340 mm. As a result of this experience, mortars were integrated as weapons in the infantry units of all armies.
During World War II a requirement developed for close-support weapons, supplementing the artillery coverage, to prevent the enemy from manning his weapons in the interval between the lifting of the artillery barrage and attainment of the objective. Since hostile forces were normally entrenched in defensive positions, plunging fire was essential. Mortars met the need for weapons that were readily transported, capable of immediate action, and suited to fire from defilade in frontline emplacements. The mortars at the beginning of the war were the small 60 mm. and 81 mm. As the war progressed, the need for larger mortars was indicated, and weapons of medium caliber, the 105 mm. and 155 mm., were provided to blast the enemy from strong points and machine-gun bunkers. Small mortars gave maximum firepower, in terms of the ratio of weight of materiel to ammunition delivered at the target, to infantry engaged in jungle and rough terrain. Based upon experience in World War II and in Korea, infantry and artillery mortars have become an essential weapon for ground troops.