Nor would failing to act in Syria mean he would let Iran off the hook. Assad allegedly used gas against his own people. If Iran gets nukes, they will be pointed outward. Assad is no threat to us; Iran might be. So the mullahs in Tehran can draw no firm conclusions from whatever Obama does in this case.
Some people think we have to uphold the powerful international taboo against poison gas to keep its use from spreading beyond Syria. But the main reason nasty regimes have abstained is not respect for global norms. It's naked self-interest.
Chemical weapons are hard to control once dispersed, making them a threat to the army that uses them. They can also provoke devastating retaliation. Saddam Hussein could have used them in 1991 during the first Gulf war. He refrained rather than invite his complete destruction. Assad had no such worry.
Any U.S. attack on Syria has clear drawbacks. One is that it's bound to kill blameless bystanders. During the famously successful air war against Serbia in 1999, the U.S. didn't suffer a single combat fatality -- but it killed 500 civilians.
There is also the risk of greater entanglement in a war we can't afford and don't need to fight. To attack an enemy and then let him survive could be seen as sowing the seeds of future trouble, not to mention encouraging other enemies.
Says retired Marine Gen. Anthony Zinni, former commander of U.S. forces in the Middle East, "If you do a one-and-done and say you're going to repeat it if unacceptable things happen, you might find these people keep doing unacceptable things. It will suck you in." Once a president puts his prestige on the line, he may find it hard to walk away.
Maybe Obama will be able to. In that case, the attack on Syria will amount to an ineffectual exercise in geopolitical symbolism. But the innocents on the ground will not die symbolically.