Many sides battles | Battle Total War: Attila Guide
Last update: 11 May 2016
Those are the battles in which more than two factions fight. In difference to battles of large armies, where both sides can control even forty formations, battles of three and more factions require different approach.
Most of all, the lack of control over allied army forces you to build your strategy based on the latter. You can't force your ally to charge at enemy or distract him when you will try to flank him. You can only adapt to what your ally will do and get most use of it.
Tactic depends on how you treat allied forces. If your ally is a partner for you and you want him to get out of battle harmless, then move your forces with his. When he attacks, help him. If he is attacked, send him backup. Treat his army as yours, just one that you cannot control and try to lead so that the losses in both armies will be more or less the same. Generally the effect should be same as in any other battle. It might be even better as you don't need to be the first to attack your enemy. You can wait until your ally is attacked and then strike from flank or back. By doing so, you will help your colleague and limit the casualties for both of you.
If ally doesn't matter to you, then you can treat his army as cannon fodder. Wait until he attacks enemy or is attacked. Then wait for the outcome. If your ally won, the better for you, battle is won with no casualties. For lack of other jobs you can hunt the survivors. If enemy won, then you must finish his remaining forces. Although you will gain less experience as your forces won't have whom to kill, but you will greatly reduce casualties in your armies.
Much depends on differences in strength. If enemy army consists of twenty formations, and you and your ally have ten of them each, then leaving your ally to himself might not work in your favor. Enemy will easily crush him and them he move at you. In that case it is better to help your partner and defeat enemy forces together. But if your ally has twenty formations, and enemy only ten, then it is wiser to back and wait for him to solve your mutual problem. You can always help him by sending cavalry at enemy back or by capturing prisoners.
Battles of for and more factions happen rarely outside battle editor. In their cases diplomatic relations between factions are irrelevant, so leaving ally to himself has no point from political sense of view. In that case not helping him simply doesn't pay off.