On the checkerboard formation

[pezhetairoi]

The triarii lines were only half the size of either the hastati line and principes, so they wouldn't have been able to make that much of a difference either. They were veterans, the equivalent of the evocati, and given that they had survived, as citizen levies, for numerous years of campaigning here or there, I suppose they had done their share of dying. Time to let the younger ones get some experience. You want to ensure you always have a pool of triarii to draw from. Also, battlefield courage gains social honour and stuff. I suppose you could argue it's so the younger ones (especially the hastati, who tend to be the youngest) get a chance to prove themselves in battle and win honours that would increase their social standing. The triarii, once again, have gotten their share in their time. But that's probably a small reason compared to other tactical considerations.

And I would suppose they didn't -hold- the triarii back per se. Once they cycled through the hastati and principes, it would have gone to the triarii. Probably, most of the time the battle was won before it came down to the triarii, so it would appear to us that the triarii were rarely used.

Furthermore, the keeping of the experienced troops for last was quite a sound consideration. After the shaken hastati and principes have retreated, and the battle is going pear-shaped, and everyone is shagged out and can't think very well anymore, you want some solid, dependable old guys to laugh at you and say, 'Hah! Running already? We'll show you what fighting is really all about!' and march forward to meet the enemy with chin held high while you stand there catching your breath, thinking 'Damn, they're really cool. Once I get my breath back I'll join them.'

Whereas if you placed the triarii in the first line and waited for them to get tired and shaken to withdraw, things would get worse. People would see the best of the army withdrawing through the gaps, with the exhausted wild-eyed look, and the older men would be going 'damn, these are real tough guys. Good fight, eh? But damn, they are tough.' Sure it would take longer for them to retreat than the hastati, for example, but they eventually would, and then you'd have the hastati and principes thinking 'damn, if even they are down, what about me?' There would be some psychological effect no matter how small. And that could grow because the triarii retreat already would give them some expectations of the enemy's prowess, perhaps exaggerated. When they go up there into the front line, surely their fears would magnify themselves. In the second line you can't really see that much and the unknown tends to make reality, when it comes, all the more frightening and disorienting. Already battle was mostly spent getting up courage. How much harder it must be to attack a bunch that have already beaten the best troops in your formation, and how much a harder job it would be for the centurions to motivate their men in the mroeale equivalent of an uphill charge.

One thing at least was that Roman consuls showed no qualms about using triarii when the time came to use them, when they were cycled to the front. Compare that to Napoleon millenia later, who lost or could not complete his victories because he kept his Garde Imperiale and refused to use them when the moment arrived, due to some mindless cherishing of them. When you keep the experienced troops for last, there are two ways it can go: if you're not afraid to use them, as and when you use them, it becomes a morale booster. Of course, it could be disastrous if those are defeated since everyone will be demoralised, but in the short term it makes everyone fight all the harder. And anyway, if you are using them in the first place, that means you're already making your last throw so if the elite lost, it wouldn't really matter if the army was demoralised anyway.

But if like Napoleon you DO refuse to use them, it hurts the army as a whole because you'd have the hastati and principes fighting and dying by droves while you're holding back the best troops, all concentrated in one formation that is never used, who in this situation would have been better used parcelled out among the frontline to stiffen the formation. The Roman legion followed the former mindset so it was really not all that bad. It wasn't particularly wasteful either, remember the imbalanced casualty rates of ancient battles. You didn't actually have -that- many people dying on the winning side. The hastati and principes normally retreated because of exhaustion, not because of casualties.

The Romans held back ALL their best troops in the rear lines because to send them all forward at once (and send them forward they did) would be an extremely visible and very powerful morale booster. To use less of them and only in certain segments of the line instead of all along it would lead to morale issues along certain areas. For them, it was a case of 'saving the best for last' rather than a fetish for collecting the whole set of triarii and keeping them elite and untouched like the Persian Immortals.