People who already understand how armies are put together should skip Part 1.
Part 1. How armies are put together
One of the things that I find irritating about battles in movie is that the director seems to think that battles are about getting an inchoate mass of soldiers together, giving a rousing speech and yelling "Charge!" That is absolutely not how it works nor ever has worked. Real armies are assembled out of groupings made from smaller groupings, themselves made from still-smaller groupings and so on down to the smallest group.
The smallest group is about ten soldiers. This is the fundamental bonding size – these are your buddies, the people you will really remember, the ones you depend on and who depend on you and for whom you will fight and sacrifice. Yes, you're fighting for Freedom or some other Large Cause, but it's really your buddy you're doing it for. So we start with about ten soldiers.
In the Roman Army this was the contubernium – a corporal, seven legionaries plus two servants who shared a tent and ate together. The fundamental tiny piece out of which everything else was constructed.
The next thing to know is the span of command or control. The commander of each level, is trying, in very difficult circumstances, to get his subordinates to do something they would never do in their right minds. They know perfectly well that the first guy in the house, the lead guy attacking the machinegun post, the first guy out of the trench, the first guy out of the landing craft is almost certain to be killed or injured. It is very difficult to get people to do this and long experience shows that a commander can only control three to five elements.
The next principle to remember is square or triangular. Armies are usually constructed by making the next level of organisation out of three or four of the lower level. Why? With three, you can have two engaged and one in reserve. (A great deal of the problem of a commander, once battle is joined, is knowing where and when to commit his reserves). The "square" structure allows two in contact, one in reserve and one resting, or two up, one in reserve and one manoeuvring. Five or six are too many but two are too few. This introduces the fundamental principles of "fire" (applying the destruction to the enemy) and "movement" (moving so as to apply that destruction most efficaciously). (Movie battles have lots of the first, but little of the last.)
Finally, we have the combat arms – infantry, armour (cavalry in its time) and artillery – and supporting arms. "Combat arms" because they directly apply the violence. Other specialities assist them: engineers help them move, transport moves them, medical patches them up, signals communicate, logistics supplies them and so on. No army can function without them.
In what follows I will discuss infantry organisations because they are the purest soldier – the other two combat arms are machines, whether tanks or guns, and the support arms are functions. But, the principles of infantry organisations are followed in the other components. It should be noted that different military traditions have different names for some of these things but it's all the same principle.
Three or four "tents" (sections) make a platoon; three or four platoons a company; three or four companies a battalion. At battalion level some specialisation will appear: it may have a mortar platoon, or a machinegun platoon, there will be a simple first aid element, some light engineers, communicators, headquarters and so on. But they are all capable of being ordinary riflemen if needed. The battalion is the first construction that is capable of some sort of independent action – it has enough companies to provide fire and manoeuvre and reserves, its machinegun or mortar elements give it some support. But it is still infantry and still pretty "light".
The next level is a brigade of three or four battalions. But there is a decision point here: do you envisage this brigade being an "independent brigade" or a sub-division of a larger formation? If the former we introduce the other arms, if the latter it remains all infantry.
An independent brigade, or brigade group, will have, in proportions depending on what you want to do, infantry, tank and artillery battalions from the "combat arms" as well as "support" elements: like combat engineers, medical and dental, post offices, laundry facilities, possibly a helicopter battalion and on and on. It is an independent military town of 4000 to 6000 people which needs almost everything a civilian town needs while also being capable of moving anywhere at a moment's notice. This formation is intended to carry out military tasks by itself with help from the air forces.
The brigade that is intended to be a piece in the next largest structure would have three or four infantry battalions and would still be mainly riflemen with very little added from the other arms. Next level is the division made of infantry, tank and artillery brigades in the proportion thought useful. In the Second World War divisions were usually the smallest thing one would see on the battlefield that could be given an independent task.
A tank division would be constructed the same way except that the basic "tent" is tank itself, three or four make a platoon, and then companies, battalions and brigades. Artillery would only rarely be organised into independent structures because while it has fire, it does not have much movement. The supporting arms – engineers, signals, logistics, medical and so on, because they exist for support, rarely appear as independent structures. In short "divisions" are infantry-heavy or tank-heavy (bitter experience has taught and re-taught that none of the combat arms can function alone).
Moving up, three or four divisions make a corps; two to four corps an army and a couple of armies make an army group.
So, a whole gigantic army group is assembled, step by step, out of our little "tents".