Obama addresses nation on Syria U.S. President Barack Obama addresses the nation about the situation in Syria from the East Room at the White House in Washington, September 10, 2013. (Reuters/Evan Vucci)

President Obama was most convincing tonight when he made the moral argument for a strike on Syria. He was least convincing when he claimed an attack would involve “modest effort and risk.”

The president took at least a measure of credit for the diplomatic initiative aimed at taking chemical weapons out of the hands of dictator Bashar al-Assad, saying he had “constructive talks” with Russian President Vladimir Putin at the G-20 meeting last week. But Obama was unable to feign much confidence in diplomacy except as a way to buy time — which he sorely needs.

He asked Congress to defer a vote on authorizing military force. As things now stand, with polls unanimous in showing that most Americans are opposed, the president would lose a vote in the House and perhaps even in the Senate. A war-weary public will not be sold on yet another war in the Middle East overnight.

If public opinion is to change, it will be because of moral outrage at the use of poison gas by Assad’s regime to kill more than 1,400 civilians, including some 400 children. “The images from this massacre are sickening,” Obama said, “men, women, children lying in rows, killed by poison gas, others foaming at the mouth, gasping for breath, a father clutching his dead children, imploring them to get up and walk.”

Obama kept coming back to the horror of the August 21 sarin gas attack on a Damascus suburb. “When dictators commit atrocities, they depend upon the world to look the other way until those horrifying pictures fade from memory,” he said. “But these things happened. The facts cannot be denied.”

At the end of his speech, Obama said this:

But when, with modest effort and risk, we can stop children from being gassed to death and thereby make our own children safer over the long run, I believe we should act. That’s what makes America different. That’s what makes us exceptional.

It was a rare endorsement by Obama — a dedicated multilateralist — of American exceptionalism in foreign policy. I believe that appeal will play well to the American conscience, especially if, as I suspect, the horrors of poison gas become a constant theme for administration officials over the coming weeks.

But the part about “modest effort and risk” hits a false note. We have all learned in the past decade is that there are no “modest” wars. The use of U.S. military force always means big risks and big consequences. The idea that lobbing missiles into a cauldron as volatile as Syria would violate this rule is ludicrous, and Americans know that.

Obama has to make the case not that an attack is cost-free, but that it’s worth it.