Dr. Michael J. Taylor

I'm going to focus on the period from 31-180 AD, mostly because our sources are the best, including two war monographs (Tac. Agricola and Josephus BJ), as well as the columns of Trajan and MA and Adamklissi.

I don't have quantification, but definitely far fewer pitched battles (i.e. where everyone lines up and fights it out, vs. sieges, petit guerre, etc.) than the Middle Republic.

It is notable that while we have a pretty good idea of what a Roman legion's deployment looks like for both the manipular legion of Polybius and the cohortal legion of Caesar, we have no idea how an Imperial legion deployed for battle.

Now, the paucity of battles has a lot to do with enemies. The Romans fight big battles with the Carthaginians and Macedonians because these are big organized states with complex armies of their own, and their own incentives to risk battle.

But then again, our best attested Imperial-era battles, at Idistaviso and Mons Graupius, were against non-state societies who still fought big battles against Roman forces.

Yes, I just ignored Kalkriese above, because I am thinking about battles where the Romans have some choice whether to fight or not. Kalkriese was an ambush.

Also, I will admit big gaps in our sources. There are certainly some big military events that we know every little about--imagine if we had a literary source of Trajan's campaigns.

So admitting that many opponents do not want to fight well equipped Romans armies in open fields, one also wonders if the Romans themselves have come to prefer modes of war outside of the pitched battle.

I am going to side-step the military manuals, and focus instead on structures and incentives for a high-risk, high-reward battle. Firstly, unlike in the Republic, where consuls need a victory NOW, emperors rule for life...

More importantly, emperors' legitimacy is a complex edifice with many social, economic and especially religious supports. Yes, emperors are supposed to be good military leaders. But it is not even the most important thing that keeps them in power day to day.

More importantly, in the principate emperors rarely command their own troops, although Trajan and Marcus point towards the soldier-emperors of the Late Empire. Which means someone else will actually command the battle for the emperor.

This flips the risk-reward calculations for the emperors' general considerably. Win too well, and the emperor might become jealous of your success, fear your as a rival, and have you killed.

This happens to poor Domitius Corbulo, who Nero liquidates for winning the Parthian War. And even if you win a big battle, like Agricola does at Mons Graupius, the emperor may not be inclined to reward your glory. Agricola is subsequently passed over for proconsular governor.

So the incentives to risk battle, in terms of political capital, no longer exist for Imperial generals. Secondly, the demographics are different as well, and oddly, less favorable to military loss than they were in the Mid-Republic. Because the Roman army is now professional

For the citizen-army of the Middle Republic, it was easy to replace losses with conscription. But Roman soldiers in the principate serve for 20 years. The loss of any soldier professional represents a huge loss of experience, and investment in his training and upkeep.

Worse, there are not many people who actually want to sign up to serve the mandatory 20 year enlistment. By the 50s AD, Italians do not want to join the legions on some distant frontier. The pool of willing recruits is increasingly the sons of soldiers, plus sundry volunteers.

This small pool of willing recruits can keep the army at strength, but they are not enough to remedy heavy losses (i.e. a lost battle). Too many casualties and you have to resort to press gangs, or even recruit, *clutch pearls* slaves and gladiators into the army.

Finally, Roman battle tactics. In the Late Republic, the Romans learn that they are far better at digging than they are at fighting (and they're pretty good at fighting). The Romans start using earthworks to reshape battlefields, basically to make battles more like sieges.

Think about many of the famous battles of the Late Republic: Alesia, Dyrrachium, Philippi. Heavy combat and heavy losses, but mediated by intensive earthworks, rather than open fields. This habit seems to keep up in the Empire. @Roelkonijn, who loves ditches, will approve.

But this quite likely changes the standard "face of Roman battle" from massed open fighting in fields to scrappy combat around earth works.

For the principate, there are no political rewards to forcing a battle, for either general or emperor. The slender base of willing recruits makes losses hard to replace, and Roman soldiers by training and disposition want to dig-rather than fight-their way out of every problem.