AMERICA is going to attack Syria, it seems, and it is going to do it because of gas. As reasons to attack a murderous dictatorship go, punishment for the use of chemical weapons to kill hundreds of civilians isn't a bad one. For anyone inclined to see America as an avenging angel of international justice, however, this fascinating scoop from Shane Harris and Matthew M. Aid at Foreign Policy will come as a bit of a downer. It seems the American government was well aware of the chemical-weapons attacks carried out by Saddam Hussein in the late 1980s, both against the Iranian army and against his own people, and not only did nothing to stop him, but in fact supplied him with the coordinates of Iranian force concentrations in full knowledge that he would use that information to poison them with nerve and mustard gas. Now, the Chemical Weapons Convention (CWC) barring nations from possessing chemical arms didn't come into force until 1997. However, the 1925 Geneva Protocol bars the use of chemical weapons in war. (Incidentally, for those who have heard that Syria isn't a party to the CWC: that's true, but it is a party to the Geneva Protocol.) So America ought to have been, if not bombing Saddam's Iraq for using poison gas, at least condemning it and applying sanctions. Instead, it supplied target data.

Apparently American reactions to violations of the international prohibition on using chemical weapons are not entirely consistent. What accounts for the differences?

The simplest explanation is just that America is willing to overlook or even abet the use of poison gas by its allies or associates, which Iraq was in 1988 when the Reagan administration was trying to contain Iran. When America's enemies use poison gas, on the other hand, it serves as a legitimate excuse to use military force against them, as in Syria today, or indeed in Iraq 15 years after its use of chemical weapons. This, however, is an unsatisfying explanation, because it implies that Barack Obama is looking for an excuse to attack Syria today, when he is plainly not. Mr Obama is visibly being dragged into war on Syria against all his inclinations and his better judgment. And you would think one lesson of 1988 is that if the American government doesn't want to punish a regime for using chemical weapons, it doesn't have to. So what is forcing Barack Obama to bomb Syria now?

One factor, obviously, is the fact that Mr Obama committed himself to treating the use of chemicals weapons as a red line. He did so at a moment when America was more disposed to intercede on behalf of the Syrian rebels (at least rhetorically, as with Hillary Clinton's "Assad must go" proclamation) than it is now; at the time, the chemical-weapons ultimatum might have seemed like a handy line in the sand, rather than the albatross neckwear it has since become. Another factor is the mounting international exasperation and sense of helplessness in the face of the carnage of the Syrian civil war. The fact that America views the Syrian regime and army as strategic enemies, whose patrons are Russia and Iran, rather than quasi-allies is certainly a necessary condition.

But the decisive factor is simply the rapid availability of mesmerisingly horrifying video imagery of the gas victims. In Iraq, video imagery of Saddam's Kurdish gas victims ultimately came out, but it took years; there was no sense of urgency or an ongoing threat. Even so, the imagery of the massacres ultimately seeded a longstanding American sympathy for the Kurdish cause and remained the clearest indictment of Saddam as a mass murderer. The impact of such video images rests partly on the unique horror of poison gas in the Western imagination.

This is not an arbitrary horror. Kinetic weaponry often kills people just as painfully as gas does, but it is theoretically connected to the military mission of seizing and holding territory, and it can be used with discretion by soldiers: to warn or deter, to aim at military targets rather than civilian ones (even if soldiers often fail to do this). Poison gas is different. It exterminates human beings as though they were vermin. It was gas, in the first world war, that became the first instantly graspable symbol of how modern states waging war treat humans as cattle or insects. The Western world's first anti-patriotic, anti-war literature emerged out of gas. It's the image of the gas-poisoned soldier coughing up his lungs, seen through the "misty panes and thick green light" of his mask, that gives Wilfrid Owen license to deny that dying for your country is a worthy or noble thing—a shift in aesthetics and values that helped trigger the grand 20th-century project (failed or incomplete, depending on your perspective) of lawful, peaceful internationalism.

Of course, bombing the regime of Bashar al-Assad without UN approval is a pretty strange and uncomfortable way to pursue the project of lawful, peaceful internationalism. Military action against Syria may be unavoidable, as this newspaper argues, but it doesn't seem very promising. We're at a rather dismal moment: we retain our horror at 20th-century evils, but we have lost confidence in our power to do much of anything about them.

(Photo credit: AFP. A handout image released by the Syrian opposition's Shaam News Network shows bodies of children and adults laying on the ground as Syrian rebels claim they were killed in a toxic gas attack by pro-government forces in eastern Ghouta, on the outskirts of Damascus on August 21st 2013.)