The United States has drawn stark lines between acceptable and unacceptable kinds of warfare -- and created a double standard in the process.

The United States announced that the use of chemical weapons by the Syrian government would be a "red line," potentially triggering intervention. The White House fears that "an increasingly beleaguered regime" may be plotting a chemical attack. Secretary of State Hillary Clinton added: "We are certainly planning to take action if that eventuality were to occur." Oddly, the international community seems less concerned by how many people the Syrian regime kills than by the methods it uses to kill them.

The rule of murdering your population is: Don't use chemical weapons. We often draw a sharp distinction between "civilized" conventional arms and intolerable weapons of mass destruction--or the evil triad of chemical, biological, and nuclear weapons.

Blowing your people up with high explosives is allowable, as is shooting them, or torturing them. But woe betide the Syrian regime if it even thinks about using chemical weapons!

A woman and her child under fire in Aleppo might miss this distinction. It's not obvious that high explosives are inherently less evil than chemical weapons. People vividly recall the horrifying gassing of soldiers in the trenches of World War I. But it was artillery shelling that killed in hugely greater numbers.

Strip away the moralistic opposition to chemical weapons and you often find strategic self-interest lying underneath. Powerful countries like the United States cultivate a taboo against using WMD partly because they have a vast advantage in conventional arms. We want to draw stark lines around acceptable and unacceptable kinds of warfare because the terrain that we carve out is strategically favorable. Washington can defeat most enemy states in a few days--unless the adversary uses WMD to level the playing field.

And Washington is sometimes willing to cast a blind eye against the use of chemical weapons, as long as it's in our interests. When Saddam Hussein gassed the Kurds in Halabja in 1988, the United States barely protested. Baghdad was seen as a secular bulwark against the more threatening Iranians--the classic lesser of two evils. Washington claimed, despite the evidence, that Iran might have been responsible for Halabja. In a tepid condemnation, the UN Security Council called on both Iran and Iraq "to refrain from the future use of chemical weapons." Washington was outraged by Saddam's behavior--fifteen years later, in the run up to the war in Iraq.

Of course, the chemical weapons taboo might still be a good idea. Whatever the motive, at least we're prohibiting some types of barbarism. But the taboo encourages double standards about humanitarian intervention based on the tools that governments use to kill.

And there are other inconsistencies in how we view civilian suffering. We can be influenced by vivid historical events. If a government slaughtered its people in concentration camps the international community would be more inclined to act. The parallel with the Holocaust would be too shocking to ignore. But if a regime massacred its people with machetes, like in Rwanda, we don't feel the same responsibility to protect.

It also matters greatly where you live. Civilian deaths in the Balkans and the Middle East capture public attention. There's particular interest if Israelis are doing the killing--or being killed. By contrast, atrocities in Africa rarely register, for example, the millions who died in the Democratic Republic of the Congo.

The combination of buttons that unlocks international interest in a humanitarian crisis can seem arbitrary. Killing with conventional weapons in Africa? Sorry, that's old news. Using chemical weapons or concentration camps in the Middle East? Now you've got my attention.