Leo has to take the President aside and talk him down, and they have a screaming row in his office, during which Bartlet makes the following statement:
BARTLET: Did you know that two thousand years ago a Roman citizen could walk across the face of the known world free of the fear of molestation? He could walk across the Earth unharmed, cloaked only in the protection of the words civis Romanus -- I am a Roman citizen. So great was the retribution of Rome, universally certain, should any harm befall even one of its citizens.
I'm not quite sure what Bartlet's thinking of here. Certainly Roman citizens had certain rights - St Paul famously appealed to be tried in Rome, and since he was a Roman citizen, this had to be allowed. Presumably if a politically important figure were to be harmed, the Imperial might would come down on whoever was thought to have done it, and actual rebellions were crushed pretty firmly. On a smaller level, if you were friendly with the local legion, presumably you could count on a certain level of protection from them, especially if you were a legionary's partner (they weren't allowed to get married, but that didn't stop them raising illegitimate families).
But I highly doubt that if you were a normal citizen and you were attacked by bandits on a road somewhere, you could protect yourself by claiming citizenship (you'd probably get an extra beating for it!). Much of the literature of the ancient world is about elite politics, or elite love affairs, or gods and goddesses, but when we do get glimpses of the rest of the world, it seems to be a pretty rough place. Apuleius' Golden Ass has several instances of people getting robbed, kidnapped, beaten and so on, and there's no implication that being a Roman citizen, as some of these characters definitely are, is of any help to them. The Jew who's mugged and beaten in the parable of the Good Samaritan (which, for some reason, my primary school thought we ought to hear every few weeks, all year round) probably wasn't supposed to be a Roman citizen, but there's no indication it would have helped him if he was.
Of course, the point Bartlet's making is that he wants to protect his own citizens by punishing anyone who hurts them. Leo has to point out to him that excessive and disproportionate retribution is, ultimately, unlikely to solve the problem. I'm not sure the Rome comparison works, though. Bartlet is motivated almost entirely by personal affection, for the late Morris, and by extension for all American citizens. But when Rome did come down hard on those who hurt Romans, it wasn't, usually, because they were feeling protective, it was because they were reasserting their power. Bartlet wants to assert his power too, of course, but he wants power because of what he can do with it, whereas Roman emperors tended to be more about power for power's sake. They were not a sentimental group of people. Augustus, for example, was proud of telling everyone how he had brought peace to the Roman world and about all the good things he'd done for the people, but he brought peace by killing everybody who didn't want him to completely destroy their democracy and take over, and brought about the deaths of any number of Roman citizens in the process.