Washington ? What does the world look like when people begin to doubt the credibility of American power? Unfortunately, we’re finding that out in Syria and other nations where leaders have concluded they can defy a war-weary America without paying a price.

Using military power to maintain a nation’s credibility may sound like an antiquated idea, but it’s all too relevant in the real world we inhabit. It has become obvious in recent weeks that President Obama, whose restrained and realistic foreign policy I generally admire, needs to demonstrate there are consequences for crossing an American “red line.” Otherwise, the coherence of the global system begins to dissolve.

Look around the world and you can see how unscrupulous leaders are trying to exploit Obama’s attempt to disentangle America from the tumult of the Middle East. As we consider these opportunistic actions, it’s easier to understand the rationale for a punitive military strike against Syria for its use of chemical weapons.

Syrian President Bashar Assad overrode a clear American warning against such use of chemical weapons. According to U.S. intelligence reports, Assad’s military last week fired rockets tipped with chemical warheads into rebel-held civilian neighborhoods east of Damascus. Reports from doctors on the scene are heart-rending. Medicine “can’t do much” to ease the suffering, wrote one doctor, because the concentration of the nerve gas sarin was so intense.

What did Assad and his generals think would happen in response to this blatant violation of international norms? Apparently, not much, and in a way, you can understand their complacency: Previous Syrian chemical attacks on a smaller scale hadn’t triggered any significant U.S. retaliation, despite Obama’s warning a year ago that such actions would be “a red line for us.”

Here’s another thought to ponder: Is it possible that the Syrian chemical weapons attack was planned or coordinated with its key ally, the Quds Force of Iran’s Revolutionary Guards Corps? Surely, they were in the loop. “After all, they’re running the show,” argues a Lebanese analyst who knows the Quds Force well.

The main rationale for military action by America and its allies should be restoring deterrence against the use of chemical weapons. The strike should be limited and focused, rather than a roundhouse swing aimed at ending the Syrian civil war. But it should be potent enough to degrade Assad’s command-and-control structure so he can’t conduct similar actions in the future. Officials hope that the strike will make a diplomatic settlement more possible; they don’t want a decapitation of the regime that would leave no counter-party for negotiation.

A second example of the dangerous opportunism that Obama has unintentionally fostered is that of Russian President Vladimir Putin. He’s a pugnacious ex-KGB officer who seems determined to take advantage of our reasonable, reticent president and the fatigued nation he represents. For a while, Putin’s chip on the shoulder was merely annoying. But in turning a blind eye to Syria’s use of chemical weapons, the Russian leader is undermining one of the precepts of the global political order.

Putin will try to exploit the fallout of American action, just as he harvested the benefits of inaction. But the Russian leader has truly brought this crisis on himself. Back in February in Munich, Vice President Biden and Russian Foreign Minister Sergei Lavrov were talking privately about the shared U.S.-Russian interest in containing Syria’s chemical weapons. Russian behavior in the months since has been selfish and obtuse, and I suspect in the long run it will prove costly to them by fostering more disorder in the region.

Obama needs to calibrate his military strike in Syria with two other regional players in mind: Iran and Saudi Arabia. The Iranians surely have read Obama’s caution (correctly) as a sign that he wants to avoid another war in the Middle East. Unfortunately, history tells us that an ambitious, revolutionary nation like Iran makes compromises only under duress. U.S. action against Assad may not deter the Iranians, but it will at least make them think twice about crossing Obama’s “red line” against their acquiring nuclear weapons.

Among Egyptian generals, Saudi princes, Israeli politicians and other conservative players in the Middle East, the consensus seems to be that Obama is a weak president — and that they need to rely on themselves for security. Obama won’t change that opinion by authorizing a retaliatory strike against Syria. But if he moves sensibly, in coordination with allies, he will at least remind people that American military power is not to be taken lightly.