Aleppo, Syria. (Narciso Contreras/AP)

It's a legitimate question: The civil war in Syria has already killed tens of thousands of people, and the regime has already been accused of killing large numbers of civilians, including children, so why does it matter if regime forces used chemical weapons in small amounts, as U.S. intelligence believes they may have? Isn't that just more of the same rampant killing that's been happening for two years? Isn't it inconsistent, and maybe a bit absurd, to say that some deaths matter more because they were caused by sarin rather than shells?

The answer is no, it's not absurd or inconsistent to suggest, as the Obama administration did when it declared a "red line" against the use of chemical weapons on Syria, that killing even just a few people with chemical weapons is somehow different than killing lots of people with conventional weapons. But the reason is about more than just Syria: It's about every war that comes after, about what kind of warfare the world is willing to allow, about preserving the small but crucial gains we've made over the last century in constraining warfare in its most terrible forms.

One of the few positive outcomes of World War I was the Geneva Protocol of 1925, in which world leaders agreed that they would no longer use chemical or biological weapons. They wanted to change not just international law but international norms, both of which were further codified by the 1972 biological weapons convention and the 1993 chemical weapons convention. The idea was that war, sadly, is going to happen. But if we can all agree not to use chemical weapons, warfare will be less terrible.

It's largely worked: With a few notable exceptions, the taboo against chemical weapons has held up. Even in some of the most vicious conflicts of the past few decades, otherwise ruthless armies and rebels have largely refrained from using chemical weapons. That's a remarkable achievement and one of the world's few successes in constraining warfare. Keeping Syrian leader Bashar al-Assad's regime from breaking the chemical weapons taboo is about more than just what happens in Syria: It's about maintaining the international norm against chemical warfare, about ensuring that present and future wars will not redeploy the awful chemical weapons that made the First World War so much worse than it would have otherwise been.

When Saddam Hussein's regime gassed the Kurds at Halabja in 1988, why was it any worse than if he had just sent in troops to kill those civilians with conventional weapons? It was bad for two reasons: because chemical weapons are an especially awful way to die (and are particularly painful for survivors, leaving many debilitated for life), and because it eroded the norm against their use.

Still, to understand what makes the norm against chemical weapons so important, you have to understand that it's chemical weapons' strengths as well as their weaknesses make them so dangerous. They're very, very useful for killing large numbers of civilians indiscriminately, but they're actually much less practical for winning conventional battles. A chemical war, then, is not just a war that kills more people; it's a qualitatively different kind of war. Their use encourages militaries to seek victory by destroying civilian populations, which becomes much easier to do when you use chemical weapons, than by overcoming the enemy military on the battlefield.

Here's what the norm against chemical weapons is not: an automatic trigger that says the world now has to intervene. As Ploughshares Fund President Joe Cirincione wrote on Twitter, "A red line is not a trip wire. Point of Obama's warning is not to start a war but to prevent a massacre like Halabja. So far, it's worked."

Nor is the red line against chemical weapons an implicit argument that wars and massacres are fine just as long as armies massacre civilians with conventional weapons. War has been happening for thousands of years; the world has been working to establish norms limiting war for less than a century, a relative blink in our history, and the chemical weapons taboo is just the start. If we let the chemical weapons norm fall back, we're also setting back any future efforts to constrain war.

One day, hopefully, we'll have norms that also make it a taboo to massacre children or put down peaceful protests with tanks, as Assad's regime has done. We're not there yet, but if we want to get there, we have to start by preserving the norms we already have. That does not necessarily mean a unilateral U.S. intervention, of course, but it does mean treating the use of chemical weapons as a transgression qualitatively different from the Assad regime's many prior abuses.