By claiming that the use of chemical weapons in Syria would be a “game changer,” President Obama seems to have boxed himself in and made intervention in Syria inevitable — or his word becomes worthless.

Anne-Marie Slaughter writes: U.S. credibility is on the line. For all the temptation to hide behind the decision to invade Iraq based on faulty intelligence about weapons of mass destruction, Obama must realize the tremendous damage he will do to the United States and to his legacy if he fails to act. He should understand the deep and lasting damage done when the gap between words and deeds becomes too great to ignore, when those who wield power are exposed as not saying what they mean or meaning what they say.

The distrust, cynicism and hatred with which the United States is regarded in much of the world, particularly among Muslims across the Middle East and North Africa, is already a cancer. Standing by while Assad gasses his people will guarantee that, whatever else Obama may achieve, he will be remembered as a president who proclaimed a new beginning with the Muslim world but presided over a deadly chapter in the same old story.

The world does not see the complex calculations inside the White House — the difficulty of achieving any positive outcomes in Syria even with intervention, the possible harm to Obama’s domestic agenda if he plunges into the morass of another conflict in the Middle East. The world would see Syrian civilians rolling on the ground, foaming at the mouth, dying by the thousands while the United States stands by.

Mr. President, how many uses of chemical weapons does it take to cross a red line against the use of chemical weapons? That is a question you must be in a position to answer.

As a Washington insider, Slaughter asserts that American credibility is on the line, without questioning the underlying presupposition: that American credibility is currently intact.

That Obama is now equivocating on the nature of his red line — that having once been defined as the mere movement of chemical weapons, the threshold is now means that their use becomes “systematic” — should hardly be treated as credibility undermined. One can reasonably argue that American credibility was in severe disrepair well before Obama took office.

The American Century lasted about two years — that was roughly how long it took for it to become plain to the world that American power was not sufficiently great that the Greater Middle East could be shaped in accordance with American designs.

One of the knock-on effects of the neoconservative demonstration of the limits of American power was that autocrats across the region who had long held on to power because that power was supposedly shielded by the United States gradually lost their own appearance of invulnerability. It was in this sense that the war in Iraq became a prelude to the Arab Spring. It wasn’t that the war unleashed democratic possibilities but rather that it became a huge demonstration of American impotence.

So, if Obama wants to live up to the image he clearly has of himself — that of the cool realist — then the pivot he needs to make (“pivot” being Washington-speak for how to elegantly eat your own words) will involve shifting attention away from “red lines” towards the capabilities the U.S. and its allies do and don’t possess. The narrative will say less about what should or must be done, and more about what can be accomplished.

To intervene in Syria because America’s reputation is at stake would be to intervene for the worst possible motive — just as driving while preening oneself in the mirror is a sure way to get into a road accident.

In as much as the intervention or non-intervention argument has been dominated by ideologues on both sides, it needs to become focused on what is possible. And it might be time to revive one of George W Bush’s earliest promises (that he never lived up to) that it is time for America to engage in the world with humility. It is humility that has by this point surely been well-earned.