Divide, Disperse and Survive - the Pentomic Division of the U.S. Army in the 1950-1960s
The U.S. Army finished the Korean War (excuse me, Police Action) and found itself in dire need of reorganization. The Army was still carrying on its business using the World War II model; unit organizations, unit heraldry, TOEs and such were all still notably unchanged. The Korean War was fought (on the ground) with a healthy surplus of gear left over from the previous brouhaha; this equipment was not only getting used up, but remaining stocks were aging - something wartime production hadn?t been intended to do well.
In addition, the world the Army faced was rapidly changing. U.S. planners were moving from a notion of ground combat on a wide scale using deployed forces to the Cold War soon-to-be norm of planning for a pivotal conflict in Europe, probably arising in the area of the Fulda Gap. This conflict would be fought using forward deployed forces and prepositioned equipment; not only that, but in the early 1950s and 1960s, there was a general acknowledgement that any conflict in Europe would likely 'go nuclear' fairly swiftly as the smaller NATO forces would be hard-pressed to hold off the looming masses of Warsaw Pact intruders. At the time, NATO's vaunted technological edge was not yet in existence; systems were still roughly comparable on each side, which meant numbers mattered - and in numbers, NATO was in severe trouble.
Nuclear weapons, tactical nuclear weapons (TNW) especially, meant that the traditional principles of organizing large-scale formations and units would no longer work. Centralizing command and support functions behind the lines, a trend which had been reinforced up to this point by the use of rail transport for supply and troop movement, was now extremely risky. A single TNW hit could theoretically destroy the entire brains and supply - headquarters and logistics - of a division or Army Group; bereft of these, the remainder of the force would quickly cease to function as a coordinated unit. Divisions, in WWII, were organized in a traditional manner, being composed of brigades, which in turn were composed of regiments. During that war, brigade-level headquarters had been eliminated in order to streamline division-sized operations in the newly fast-moving motorized and armored conflict. As a result, three brigades (up one from two before that) as well as a full complement of combat support and combat service support (CS/CSS) units all resided under the command of a single divisional HQ and supply arm. With the destruction of these central elements made much easier by TNW, the durability of these formations had dwindled rapidly.
The solution, as the U.S. Army saw it, was to encourage smaller, more independent units. Since the battlefields in the theater were expected to be extremely lethal and broken (nuclear conditions, the mountains and hills of Europe), smaller units would be expected to fare better when the division's unity was struck by atomic attack. As a result, active duty units were transitioned to a force structure called the Pentomic Division - so named for being split into five roughly-independent fighting units. The premise was that in the event of TNW being released onto a battlefield - or in the event of the division suffering a breakthrough (like the Blitzkrieg technique, or the Battle of the Bulge) the various (remaining) parts of the division would still be able to fight with minimal loss of efficiency. If it suffered a TNW attack, there would no longer be a single critical 'center of gravity' vulnerability for the entire structure.
There were, naturally, tradeoffs. Since smaller units were now expected to cope with their logistics and CS/CSS themselves, the number of men in each fighting unit dropped. Whereas before the battalions were composed primarily of tooth, the additional integral support assets meant that each of the new, resulting units (called 'battle groups') had a larger tail. The total divisional size dropped as well, as the elimination of now two layers of command (brigade had gone in WWII, and the Pentomic concept dropped the longest-standing Army structure, the Regiment, as well) meant that the divisional command was faced with a greater coordination problem than before. Also, the emplaced nature of these forces caused support assets to be parceled out more thinly; divisions were expected to make use of the vast and growing infrastructure of logistics available in Western Europe to compensate for their own, smaller battle group based CS/CSS units.
The Pentomic Division was a single-purpose unit; to fight and survive in a nuclear environment on the NATO/Warsaw Pact Central Front. By the end of the 1950s, the concept was beginning to prove unworkable; the non-nuclear, overseas-deployed requirements of Vietnam were beginning to inform U.S. military organizational planners. The introduction of the helicopter in large numbers produced the Airmobile and Air Assault division types, neither of which fit the Pentomic mold. Finally, strict bean-counting kicked in; commentators and pundits noted that NATO and the Pact were fond of comparing 'numbers of divisions' - but that the Pentomic divisions were actually several thousand men short of their Pact counterparts in size, further exacerbating a problem of perceived preponderance on the Pact side.
Beginning in the 1960s, the Army began to transition to its (current) scheme, based on the armored division 'combat command' structure. Combat commands were renamed back to brigades, and the same command/organizational structure was put in place across all divisions, simplifying operations. The divisions became identifiable by their weight and type; the Heavy divisions were Armored and Mechanized divisions, and the Light were Infantry, Airborne and Air Assault/Airmobile.