Last night I fought a battle in Rome: Total War - Barbarian Invasion that I found to be a suprisingly good device for the purpose of illustrating many real-world tactical concepts. The fact that it was a hard-fought one particularly enhances its value in this regard.
Here, the Goths (red) deployed in a single massive line of foot with the horse evenly distributed on the wings. My slightly larger force of Franks, on the other hand, formed in multiple lines. The first line of foot was made up of cheap spearmen. In the second line stood the heavy foot, including a unit of ex-Roman mercenaries. Foot archers made up the third line, and the horse (much more numerous than the enemy's) stood in reserve to either flank and behind this third line.
My original plan had an offensive and a defensive version. If the Goths chose to wait out my attack, I would engage one wing of their foot with my first line while maneuvering my reserves to hit them in the flank. On the other hand, if the Goths chose to move first, I intended to absorb the shock of their initial assault with my first line while bringing the reserves to bear upon their least well-guarded flank. As it turned out, the Goths attacked first.
The Goths' assault followed a simple but effective scheme. They brought the bulk of their force--horse and foot alike--against my left wing while a small diversionary force pinned down my right. As soon as the lines clashed I marched my horsemen around both of their flanks with the intention of striking the finishing blow. But the plan did not work as well in practice as it did on paper.
Lo and behold, the Gothic onslaught tore my left wing apart before my horsemen could move in to smash them. The rout of the first line there exposed my third line of archers to the victorious Goths, and soon its left wing broke as well under the ferocity of the attack--although not before it had held for some time and bought me a few precious moments in which to maneuver my troops.
My center had remained largely unengaged, so I swung it to the left barely in time to prevent the Goths from outflanking and overwhelming the entirety of my line. My heavy infantry reserve also rushed around and struck the Goths in the flank. This stopped them for the time being but failed to rout them. Meanwhile, I remembered that I had a superiority in cavalry so I marched the rest of my horsemen into position, ready to fall upon the Goths when I tell them to.
On the right wing my fortune was somewhat better, since the Goths' diversionary force was too small to make any impression upon my line. Soon my right wing had routed them and opened a way for the heavy infantry reserve to commit the desperate rush I described above. The small force of horsemen I originally sent to outflank the Gothic diversionary force turned out to be unnecessary, and I nearly forgot that it even existed. The next time I looked I found it just holding its own against a much larger force consisting of the Goths that had not joined the main battle.
Finally, with the Goths' main striking force all set up, I brought my horsemen down upon their flanks and rear. I also just managed to extricate my small contingent of horse on the right wing and bring it over to aid the major assault. This move soon routed the bulk of the Gothic force and killed both of their generals.
Now that the Gothic force has been routed, I sent most of my horse into the pursuit. The rest swung around and crushed the remnants of the Gothic horse who had tried to catch up with the fighting. Some of the Gothic foot still remained unbroken and tried to march away, but some of my pursuers caught up with them and dispersed them in a brief and bloody series of charges.
In the end, I lost more than a third of my force but managed to hold out and practically annihilate the Goths in return. There are several points of tactical interest I can extract from the battle:
1. The Goths made an effective application of overwhelming mass to crush my left wing. As you can probably guess, this is an example of the principles of Mass and Offensive in action.
2. In such a large battle, it is easy to lose track of the detached parts of the army. In the momentary panic caused by the breaking of my left wing, I forgot that I had a large mass of horsemen already poised to strike the Goths' right flank. I also forgot that I had sent out a small group of horsemen out ahead of my right wing, which resulted in that group being nearly overwhelmed by the enemy. The lesson? Writers, remember that it's impossible for a single commander to keep track of everything in a battle. Some things will inevitably get missed and forgotten. The presence of competent subordinates that I could trust would probably have alleviated much of this problem, but Rome: Total War did not really allow me to simulate this.
3. My decision to keep a large part of my foot in reserve turned out to have a dramatic effect on the outcome of the battle. Without this reserve, it's hard to imagine how I could have held the Goths back after their initial success--so the reserve was a succesful application of the principle of Security. Moreover, it allowed me to regain the initiative and return to the Offensive. So, no matter how many troops you have, it is always a good idea to keep a reserve in battle. (Except in a modern naval battle, which demands a very different application of the same basic principles.)
4. It's easy to see how the neat battle-lines at the beginning of the fight soon degenerated into a shapeless mess. A good general is one who can see the order (and the opportunities) hidden beneath this apparent chaos.
By the way, it might be a good idea to take a look at another post explaining the basic application of the tactical principles mentioned above in order to get a clearer picture of them and their definitions.