Israel is a democratic state that is trying to respect human rights while defending itself against an adversary, Hamas, that does not. Benjamin Netanyahu, Israel's prime minister, is obviously right to say that the killing of Palestinian children is something Hamas desires as part of its strategy.

My sympathies are therefore entirely with Israel, and against not just Hamas but those Westerners who fling terms like "genocide" against the Israeli government.

Yet even just causes can lead their partisans astray. Increasingly I'm seeing supporters of Israel make the argument that there are no true "civilians" among Palestinians - that because many of the residents of the Gaza Strip once voted for Hamas, they're essentially participants in its terrorism.

That argument is wrong and dangerous, and it ought to be repudiated.

At a recent pro-Israel rally in New York, Rabbi David-Seth Kirshner said, to cheers, "When you are part of an election process that asks for a terrorist organization which proclaims in word and in deed that their primary objective is to destroy their neighboring country and not to build schools or commerce or jobs, you are complicit and you are not a civilian casualty."

Thane Rosenbaum, a novelist and legal scholar, wrote similarly in The Wall Street Journal: "On some basic level, you forfeit your right to be called civilians when you freely elect members of a terrorist organization as statesmen, invite them to dinner with blood on their hands and allow them to set up shop in your living room as their base of operations."

He allows that some Gazan casualties must have been civilians, and some must have been "children whose parents are not card-carrying Hamas loyalists." He concludes that "our sympathy should be reserved" for those victims: "The impossibility of identifying them, and saving them, is Israel's deepest moral dilemma."

And in Haaretz, the historian Benny Morris complains that Israel has shown "no willingness to exact a heavy price in blood from the enemy's civilians."

If we don't hold a firm understanding of why civilians should be treated differently than combatants, we can end up in indefensible positions.

I see no way to interpret Rosenbaum's comment except to mean that we should have no sympathy for children who are killed if their parents were Hamas loyalists. That's a disgusting sentiment.

The just-war tradition of thought treats civilians differently in that just military action can never be designed to kill them.

Their killing can only be tolerated, in that tradition, as an unintended byproduct of military action - and that evil can be tolerated only if the military action is likely to accomplish a good proportional to it. (It may be that Morris, his infelicitous phrasing aside, doesn't intend any inconsistency with that tradition. Other remarks in his op-ed suggest that he merely means that Israel should be willing to tolerate more Palestinian civilian casualties, rather than that it should welcome more of them.)

This intellectual tradition isn't based on sentimentality about civilians. Its conclusions don't turn on any judgment of civilians' political or moral beliefs, which in many cases may be odious. Civilians may even be complicit in some sense in the evils of war - for instance, by voting in a war-making government. Yet they're noncombatants because they're not involved in warfighting in any direct way, and their killing can't be justified as an extension of the principle of self-defense.

Thus, American civilians can't justly be targeted for acts of war that their government conducts overseas. They don't become combatants even if many or most of them vote for politicians who start wars, even unjust wars.

One point of having rules of war is to impose a degree of separation between judgments about which wars are justified and judgments about what can be done in them - so that people and governments who disagree about the justice of a war can nonetheless agree on norms about its conduct.

Supporters of Israel often say it represents the cause of civilization: of human rights and freedom.

The rules of war are a civilizational achievement worth defending.

We shouldn't throw them away, or throw away their moral underpinnings, for ill-considered theories of collective guilt.

Ramesh Ponnuru is a Bloomberg View columnist and a visiting fellow at the American Enterprise Institute.