Part 1

The short answer to your question is that there are a lot of variables in determining the appropriate frontage for an infantry division in a given locale during the Second World War, but that in general, in a defensive posture, a US Army infantry division was expected to cover (with one regiment in reserve) 10,000 yards at most, while in an offensive posture and conducting an attack this was reduced to 4,000 yards at most. Minimum frontages in each scenario were in the neighborhood of about 4,000 yards and 2,000 yards respectively.

The longer answer, of course, is complicated. There are a lot of numbers thrown around in a few different manuals, specifically FM 7-20, Rifle Battalion (1942), FM 7-20, Infantry Battalion (1944) and FM 7-40, Rifle Regiment (1942). Because the FM series manuals were the Army’s handbook, so to speak, they cover every basic military operation recognized at the time of their printing, including how to conduct an effective defense and how to attack in a variety of settings. Your inclusion of terrain and weather considerations in your question is key, because both of these factors, particularly terrain, have very serious and even decisive effects on how a given formation would deploy itself.

Generally speaking, a unit wants to deploy itself so that it can see or shoot whatever moves across its front and flanks. In open terrain it can spread out over a considerable distance without compromising that ability, while in broken or wooded terrain it cannot, so it must remain constricted so as to adequately cover its sector. In an attack a unit strives to achieve a degree of concentration that will allow it to bring the most of its firepower to bear on an enemy while retaining as high a degree of maneuverability as possible; therefore, it remains concentrated, but not so densely packed that its subunits cannot maneuver within their own battlespace. In open terrain this concentration can be achieved over a larger area, but in broken or wooded terrain it cannot.

The complication in answering your question comes in determining how a unit was disposed. In general, any commander seeks to retain a reserve, a sizable portion of his forces that he (or she) can use to influence the battle once its already joined. During the Second World War the maneuver elements of the US Army infantry division were organized in a triangular fashion, that is, three rifle squads in a rifle platoon, three rifle platoons in a rifle company, three rifle companies in an infantry battalion, three infantry battalions in an infantry regiment, three infantry regiments in an infantry division. The division also included supporting and heavy weapons at the company, battalion, and regimental level as well as four battalions of field artillery and, often, attached tank and tank destroyer battalions under division control. At any given level it was understood that the combat commander would strive to retain one of these three subunits, be it a platoon at company level, a battalion at regimental level, or a regiment at division level, as a reserve. Therefore, when you read these manuals, you have to read them carefully, because the way they’re worded can be confusing. Case in point, in the 1942 edition of FM 7-40 its stated that the frontage for a regiment in defense of open terrain should not exceed 5,000 yards. It’s easy to miss that this is a prescription for a regiment retaining one battalion in reserve, which is stated in the line above, while a few lines below it states that a battalion may be assigned a frontage of as much as 2,500 yards in open terrain. It does not specify if that battalion retains a company in reserve, and in the 1944 edition of FM 7-20 it is stated that a rifle company in open terrain may be assigned a frontage of 1,000 yards or more. So what’s “the truth” considering that you can’t get to 2,500 by adding increments of 1,000?

The truth is that these are best practice numbers. If you’re a battalion CO assigned to cover an open plain in a defensive posture and your command is at full strength, you can spread it out over 2,500 yards and still retain a company in reserve by effectively siting your heavy weapons. You can get in touch with the people in charge of the units on your flanks (if there are units adjacent to you) and ask them what their dispositions are and act accordingly; maybe there’s a section of your line that needs to be covered by people, and the guys on your left are dug in in such a fashion that you can cover a significant portion of that flank by fire using your heavy machine guns. By positioning your people in such a fashion you can extend your frontage without tapping into your reserve.

Let’s continue with this scenario. Let’s say that your battalion is part of a division with two regiments in the line and one in reserve. Your parent regiment is covering open ground, retains one battalion in reserve, has your battalion and another in the line and both you and the neighboring battalion are covering 2,500 yards while retaining a company in reserve. The other regiment is deployed the same way, except over a series of wooded hills and gullies that hamper its ability to effectively cover its sector. Both of its battalions can only cover 1,000 yards. Therefore, the division is covering 7,000 yards. As your division moves this frontage is going to expand and contract according to the terrain its operating over, the losses it incurs in combat and what reinforcements it receives, and what job it is performing.