The assault began suddenly. There I was, quietly sipping my espresso at the Edge of Night, when my solitude was taken hostage. "Watch your fingers, Professor," came a voice from behind. Don pushed a table against mine and Zack pulled up four chairs. Julie and Theresa appeared from nowhere. Still standing, Zack flagged down the waitress, saying "One Road Kill Special, with fries." Don threw in "Same here, but supersize the fries." Zack said "Mine too."

Julie rolled her eyes. "Are those two competing or something?" Theresa commented "Honestly, you'd think they'd never seen food before." She and Julie ordered small pasta salads.

Don got the point. "Sorry," he said. "Very rude," Zack agreed. "We should have let you order first." They offered to fast from their Strombolis. "Never mind," said Theresa. "We'll assign suitable penances later."

Don turned to me and said, "You don't mind our invasion, do you, Prof?"

"It seems to be a fait accompli," I replied.

"Say," he said. "On the way here we were talking about terrorism. Since you're in a not-minding mood, do you mind if we pick your brains?"

"Consider them ripe and ready. What about?"

Theresa said, "Terror. The anniversary of Nine-Eleven was this week. We've all had conversations with friends, and a lot of the same issues keep coming up."

"More to the point," said Zack, "the same conversation stoppers."

"Right," Don agreed. "Like today. Zack and I were down at the Union, talking with a guy we know named Jason."

Zack grinned. "Jason's an expert in conversation stoppers."

"Anyway," said Don, "At first we talked about the causes of terrorism. Then we talked about what you can do about it. All three of us joined in. We seemed to be going fine until Zack said — how did you put it?"

"I said that for me the big question is how to fight terrorists without turning into a terrorist yourself."

"What happened then?" I asked.

Don answered, "Jason said, 'Get real. You do whatever you can. The only difference between a terrorist and a freedom fighter is whose side he's on.'"

"How did you answer him?"

They looked at each other. "He didn't give us a chance," said Zack. "He just said 'I gotta go' and walked off."

Julie broke in. "I haven't been walked off on, but I've heard pretty much the same thing from other people. Like in the night course I'm taking."

"I thought you were taking my advice about taking a break from school."

"I am," she said, "but I'm off work on Thursdays — how else could I be here? — and anyway it's a non-credit course. Well, someone mentioned the 'war on terror.' A girl in the class said 'All war is evil. Terror is just a word we use for what the other side does in a war."

"Did you answer?"

"Yes! I said 'If that were true, we couldn't ever criticize our own side. We couldn't make distinctions. But I can criticize my own side. I can make distinctions. Like in World War II, when we firebombed Dresden. Our cause was just, but that was wrong."

"What happened next?" I asked her.

"I ran out of steam," she confessed. "The instructor asked, 'You say you have standards, but aren't all standards merely one's own standards? Aren't they culture-bound?" I didn't know how to answer that. I felt like he'd played a trick on me, but I couldn't see quite where."

"I think I see where," Theresa said. "The proof is in the pudding, isn't it? Julie's instructor should have asked her what standards she had in mind. Demanding an abstract demonstration that not all standards are culture-bound was just another conversation stopper."

"Um," Julie said, "That sounds great, but I didn't exactly say that I have standards. What I said is that I make distinctions."

"Don't you have to have standards to make those distinctions?" asked Don.

"I suppose I must, but I don't know what they are."

"What kind should you have?" I said. "What you're trying to do is show that terrorism isn't just a word we use for what the other side does in a war. So you're really looking for standards for — what?"

"War in general," said Zack.

"Right. Now we don't want to reinvent the wheel. Have standards already been worked out for war in general?"

"Isn't that what just war doctrine is supposed to provide? I've heard you talk about it. Julie's instructor would reject it because its origin is Christian. But its standards are supposed to be general moral rules that any fair mind should be able to recognize as right — isn't that the idea?"

"It is. Standards like what, though?"

"Like, 'there has to be a just cause'? That's the only one I know."

"It sounds pretty empty," said Don. "I hate to sound like Julie's instructor, but everyone thinks that his own cause is just."

I answered, "'There has to be a just cause' doesn't mean 'Any cause that I promote is just.' It means that war shouldn't be waged except to protect innocent life, to ensure that people can live decently, and to secure their natural rights."

"I don't think terrorists are particularly interested in protecting innocent life," Julie said. "Look at the way that they fight."

"We can come back to how they fight," I said, "but not so fast. 'Just cause' isn't a criterion for how to fight a war. It's a criterion for whether it's all right to go to war in the first place. Let's finish those criteria first."

Don said, "Finish them? You mean 'just cause' isn't the only criterion for whether a war would be just?"

"Certainly not. For example, there has to be 'competent public authority.' That means that war must be declared by a legitimate government. It's not something to be decided upon by vigilantes."

"I know that terrorists violate that standard," said Julie. Take al-Qaeda. They don't represent any government."

"True," said Don, "but al-Qaeda's not the only game in town. What about state-sponsored terrorism? Would we say that because it's sponsored by a state, it's not terrorism?"

"The criterion refers to a legitimate government," Theresa pointed out. 'Is every state legitimate?"

"Does legitimacy require democracy?" asked Zack.

"Julie's instructor would say that legitimacy is in the eye of the beholder," Don protested.

"Yes," Julie said, "but Julie's wristwatch says Julie has to get back to work soon. Professor Theophilus, could you skip the rest of the when-to-go-to-war criteria, just give the others quick answers to their questions, and go back to the other criteria you mentioned — the ones about how war has to be fought?"

"I could do that. Are the rest of you agreed?" They nodded. "All right, quick answers. Anyone who doesn't like them can argue with me later in my office." Zack grinned. "Theresa, you're dead right that not every state is legitimate. Don, it's true that evaluating a state's legitimacy requires a moral judgment, but that doesn't make it arbitrary. For example you can ask whether the rulers are motivated by their selfish interests or by the common good. Zack, democracy is good if you can manage it, but a state can be legitimate without being democratic, and it can be democratic without being legitimate. Suppose that a corrupt majority wants to murder the innocent minority! Am I hurrying fast enough, Julie?"

"Yes," she said. "But keep your promise!"

"Right — about the war-fighting criteria. The first is 'proportionality.' We must never use tactics that can be expected to bring about more evil than good."

"Another of those slippery judgment calls," said Don.

"The second is 'right intention.' Even in war, our goal shouldn't be to annihilate the enemy, but to achieve a just peace. That means we have to avoid any act or demand that would make it impossible for them to reconcile with us some day. Of course they may nurse hatreds of their own that would prevent reconciliation, but those aren't our responsibility."

Theresa said, "You can't call that one slippery, Don. Terrorists don't believe in reconciliation. They don't even pretend to."

"The final criterion," I went on, "is 'discrimination.' If you want to understand terrorism, that's the crucial one."

"Discrimination between what and what?" asked Julie.

"Between combatants and noncombatants," I said, "between soldiers and innocent bystanders. Of course harm might come to people accidentally, but directly intended attacks upon innocents are categorically forbidden."

"Why do you call that criterion the crucial one?" asked Zack.

"Because it's the one that shows most clearly that terrorism isn't just a word we use for what the other side does in a war."

"Don't you see, Zack?" Theresa broke in. "Killing innocents is the whole point of terrorism. Kids in school buses. Mothers in supermarkets. All those people in the Towers. None of those people were combatants."

"Don said, "Are you saying that terrorists believe in murder? That they don't think that it's evil?"

"They murder because they know that it's evil," I answered. "According to terrorist theory, that's why it works; only moral abomination is traumatic enough to achieve the result that they seek. They view deliberate destruction of innocent human life as a way to bring down a government that they aren't strong enough to defeat directly. The goal is to alienate the government from the citizens — not only by proving that their government can't protect them, but by driving the government to more and more repressive measures in the attempt to stamp them out. Ideas like these were circulating long, long before anyone heard of al-Qaeda."

Zack was thoughtful. "So when Don and I were talking with Jason, I asked the right question after all. How does a nation of people fight terrorists without turning into terrorists themselves?"

"Keep asking that question," I said. "One side is betting that there's an answer. The other side is betting that there isn't."