As the US approached entry in World War II, armored employment was doctrinally governed by FM 100-5 Operations (published May 1941, the month following selection of the M4 tank's final design). That FM stated that: The armored division is organized primarily to perform missions that require great mobility and firepower. It is given decisive missions. It is capable of engaging in all forms of combat, but its primary role is in offensive operations against hostile rear areas.
In other words, the M4 was envisioned to primarily fill the role of a cruiser tank — although the US Army did not use that doctrinal term. The M4 was not primarily intended as an infantry support tank; in fact, FM 100-5 specifically stated the opposite. It placed tanks in the "striking echelon" of the armored division, and placed the infantry in the "support echelon". Neither was the M4 primarily intended for tank versus tank action. Doctrinally, anti-tank engagements were the primary role of tank destroyers. The field manual covering the use of the Sherman (FM 17-33, "The Tank Battalion, Light and Medium" of September 1942) devoted one page of text and four diagrams to tank versus tank action (out of 142 pages). This early armored doctrine was heavily influenced by the sweeping initial successes of the German blitzkrieg tactics. Unfortunately, by the time M4s reached combat in significant numbers, battlefield demands for infantry support and tank versus tank action far outnumbered the occasional opportunities for cruiser tanks.
Although envisioned primarily as a cruiser-type tank, US doctrine did also contemplate the M4's use in other roles. Unlike some other nations, which had separate medium tank designs tailored specifically for anti-tank roles (e.g., the German PzKw III) and support roles (the PzKw IV), the US intended the M4 to fulfill all roles. Although not optimized for tank versus tank engagements or infantry support, the M4 was capable of performing these missions to varying degrees. In the Pacific Theater, the Sherman was used chiefly against Japanese infantry and fortifications; in their rare encounters with lighter Japanese tanks with weaker armor and guns, the Shermans were vastly superior.
The doctrine of the time had Shermans as a sort of infantry tank. All anti-tank work was supposed to be done by tank-destroyer crews. Speed was essential in order to bring the tank-destroyers from the rear to destroy incoming tanks. Thankfully, for Sherman crews, this doctrine was not entirely used as it would create a small window of time of weakness in the armored battalion until tank destroyers moved to the front. Obviously this would make it harder for an armored force to achieve a breakthrough, a main objective of armor, if the enemy had tanks. It would also be easier for an opposing armored force to achieve a breakthrough against an American tank battalion which would not have all of its anti-tank assets at the front during the beginning of any attack