Yesterday’s admission by the White House and Defense Secretary Chuck Hagel that Syria appears to have used chemical weapons against its own people took the debate about American policy toward the embattled Assad dictatorship to a new level. There are still no good choices available to the United States since the rebels fighting the regime are, at best, a mixed bag, and if successful may bring Islamists to power in Damascus. But, as I noted previously, President Obama’s preference for “leading from behind” and simply sitting back and hoping for the best won’t work. Allowing an Iranian ally to use Sarin gas to commit mass murder without lifting a finger to stop it is both morally wrong as well as bad geostrategic policy. So too is a policy that would not give the U.S. the leverage to help those Syrian rebels who are not Islamists prevail over those who are extremists.

But there is another angle to the decision that the administration will have to make on Syria that has wider implications for the region. With even ardent Obama supporters like Jeffrey Goldberg reminding the president he has made it crystal clear that chemical weapons use would be a red line that would trigger a strong U.S. response, what follows will not only tell us whether that promise would be kept. It will also illustrate just how seriously to take other pledges the administration has made, specifically its vow never to allow Iran to go nuclear. With the White House desperately trying to buy time before making a decision on Syria, it’s fair to ask why anyone should regard American rhetoric on Iran as anything more than an elaborate bluff if Obama won’t keep his word about Assad’s behavior.

Judging by the reaction in Washington to the news about the proof of the Assad regime using chemical weapons, many in the administration may now regret the president’s willingness to make promises about Syria. It is likely that he and his foreign policy team naively believed that Assad would fall long before they were called to account for their loose talk about being willing to act if the dictator went too far in trying to preserve his regime. Moreover, having largely been propelled into office by American war weariness, it will be difficult for a president who prefers to lead from behind to convince his supporters to back American involvement in Syria.

But if after his trademark slow decision-making process unfolds, the president decides that the U.S. will still not do anything to prevent the future use of chemical weapons or to limit Assad’s ability to use them, a crucial red line will have been crossed.

As I have written many times, its more than clear that the ayatollahs neither respect nor believe President Obama’s many warnings about their nuclear ambitions. The president’s record of dithering, feckless engagement policies, slowness to enact and then enforce economic sanctions and his commitment to a diplomatic process that has repeatedly failed have given Tehran good reason to doubt that he means business about the issue or that he regards force as an option that is truly still on the table.

Yet the president’s continued use of strong words about Iran leaves open the possibility that they are wrong. But if he cannot muster the will to do something about Syria even after the death of 70,000 people at Assad’s hands and the use of chemical weapons, then why should anyone think Obama will ever act against Iran?

That places the Syrian decision in a context in which the possible costs of inaction are far greater than the justified worries about those of intervention. There may be no good options in Syria, but the blowback from a realization that the U.S. won’t stop mass killings in this manner may be far more costly. The price may not be paid by Americans, at least not immediately, but the toll in blood and diplomatic and security complications will be great. If American “red lines” mean nothing, then Obama’s blind faith in diplomacy will be exposed as a disastrous sham.