Pentomic (cf. Greek pent(e)- + -tome "of five parts") was a structure for infantry and airborne divisions adopted by the US Army between 1957 and 1963 in response to the potential use of tactical nuclear weapons on future battlefields. It was intended that the five subordinate units, which were often referred to as battle groups (to distinguish them from traditional units), would be able to deploy and engage in operations more rapidly than conventional brigades while also having greater offensive capabilities than conventional battalions.

One US Army publication defines the pentomic division as "a public relations term designed to combine the concept of five subordinate units ('penta') with the idea of a division that could function on [either] an atomic or nonatomic battlefield".

Several other countries also temporarily adopted similar structures in their armed forces, at around the same time as the US example, including France (from 1955), Australia, Turkey, and Spain.

The addition of mechanization to army forces led to rapid changes in doctrine. During World War I the defensive firepower of infantry forces and especially their associated artillery made manoeuvrability almost impossible without overwhelming numbers. Any breakthrough could be countered by reserve forces that moved at the same speed as the attacking forces.

With the introduction of the first tanks, much smaller forces could effect a breakthrough, and move much more rapidly than the defending infantry. Ideally, this would force the defenders to retreat to new lines in order to maintain a front line. By the start of World War II, this basic concept had developed into the idea of a "spearhead", a dense formation of highly mobile forces that would concentrate at a single point, overwhelm them locally, and then run into the lightly defended rear areas. This became known as Blitzkrieg after its initial successful employment by the German forces.

Nuclear weapons dramatically upset this concept. In a nuclear battlefield, the concentration of forces into a spearhead would present a perfect target for the employment of tactical nuclear weapons. A single well-placed weapon could break up the attacking forces before they even had time to properly prepare, causing enough casualties to make them ineffective even in the defense. In the battles foreseen by planners in the 1950s, traditional infantry and armored units appeared to be extremely vulnerable.

It was this weakness that led first to the New Look of 1953, and then to the "New" New Look of 1955. The latter, especially, aimed to counter any Warsaw Pact action in Europe with the use of nuclear weapons on the battlefield, allowing NATO's superior airpower to destroy the Warsaw Pact's massed armor. As part of this shift in policy, the majority of US military development and funding was sent to the US Air Force and US Navy; the Army was, to a degree, ignored.

In July 1955, General Maxwell D. Taylor became the Chief of Staff of the United States Army and selected General William Westmoreland as his Secretary to the General Staff. Westmoreland recalled that Taylor was told by President Dwight Eisenhower that he had to do something to give the Army "charisma"; something in Westmoreland's words to give the Army a "modern look".