Left, then-Secretary of State Colin Powell holds a vial of anthrax before the United Nations in 2003. Right, a Kurdish woman visits the graves of family members who were killed in a 1988 chemical weapon attack by Saddam Hussein. (Getty, AP)

Behind the question dominating Western discussion of the escalating violence in Syria – how to respond if Bashar al-Assad's army uses its chemical weapons? – is another question, just as fraught and perhaps even more pressing: Where is the red line? In other words, does the United States act when it believes Assad has decided to use the weapons, or wait until he deploys them? How can it tell either of these for sure? If he prepares them for deployment, is that enough to mandate action? If the world waits until after he's used them, isn't it too late at that point?

The United States's challenge on Syria's chemical weapons is to minimize the possibility of two distinct nightmare scenarios, both of which, problematically, could require very different paths to avert. Both also echo of some of the darkest moments in neighboring Iraq, the legacy of which still weigh heavily on the United States and its approach to the region. The first, the one we're hearing about most, is that Assad uses his chemical weapons, despite all the warnings. The second, perhaps just as scary, is that the United States acts too preemptively, escalating or even entering Syria's war unnecessarily.

For that first scenario to happen, the United States would have to fail in its ongoing effort to scare Assad out of using chemical weapons, and then fail to react quickly enough to stop him if he decided to go ahead anyway. Fortunately, the more the United States does to prepare to act to secure the chemical weapons, the less likely it will have to follow through, assuming that Assad's regime remains rational and self-interested. This may be why Secretary of State Hillary Clinton has stressed that "this is a red line for the United States," warning that "suffice to say we are certainly planning to take action."

The United States cares about that red line for reasons extending beyond this wars and this country: the taboo against chemical weapons helps keep them out of conflicts where they might otherwise be used. As long as war is a feature of human existence, the world has an interest in keeping chemical weapons – which can disproportionately effect civilians, further increasing both the civilian death toll and the long-term health impacts – out of those wars. White House national security spokesperson Tommy Vietor told Time as much, explaining of the chemical weapons red line, “The international community has spelled out a specific set of rules and norms outlawing the use of chemical or biological weapons. The death to civilians is indiscriminate and the human suffering they inflict is horrific.”

But the U.S. record on enforcing this norm is not perfect. It's still not clear what exactly happened during the brutal 1980s war between Iran and Iraq, but human rights groups have accused the United States of turning a blind eye when Saddam Hussein, then an ally, used chemical weapons against Iran as well as his own people. Iraq's Al-Anfal campaign, in which the country deployed chemical weapons as part of an effort that killed as many as 180,000 ethnic Kurds, almost five times as many people as have died throughout Syria's 20-month war. It's impossible to know for sure what the introduction of chemical weapons would mean for Syria's conflict, but Al-Anfal provides a chilling lesson in how dramatically the weapons could worsen an already high death toll.

But there's another Iraqi lesson for the U.S dilemma on how and when to respond to Syria's chemical weapons. On Feb. 5, 2003, then-Secretary of State Colin Powell raised a vial of anthrax at the United Nations Security Council, highlighting Iraq's thousands of liters as a major plank in the U.S. case for war. Some U.S. officials suggested that the Iraqi anthrax may have been used in the 2001 anthrax-filled letter that showed up at the U.S. Senate. The Department of Justice actually ended up investigating an American scientist for the attack, and was on the verge of pressing charges when he died by apparent suicide.

Powell also displayed spy photos of four "chemical bunkers" as evidence of Saddam's dangerous chemical weapons arsenal. And, about a month later, the United States was leading the ground invasion of Iraq that devolved into a bloody, years-long war. But it quickly turned out that the chemical bunkers were empty, and a 2004 report concluded, "Iraq unilaterally destroyed its undeclared chemical weapons stockpile in 1991." The chemical weapons fears that had kick-started the war turned out to be over-hyped.

That scene of Powell at the United Nations, now almost a decade old, may be playing through the minds of U.S. officials as they consider reports on Syria's chemical weapons. On Wednesday, NBC News reported, citing unnamed U.S. officials, that "The Syrian military is prepared to use chemical weapons against its own people and is awaiting final orders from President Bashar Assad." No other major outlets have since made similar claims. Whether or not they ever do, and whether or not U.S. officials come to believe that Assad is really preparing to cross that U.S. red line on chemical weapons, the lessons of Iraq will likely weigh heavily.