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“At what point do we say we need to confront actions that are violating our common humanity? I would argue that when I see 400 children subjected to gas the moral thing to do is not to stand by and do nothing,” he said. “This is the part of my job that I find most challenging. I would much rather be talking how to make sure every 3- and 4-year-old gets a good education than I would spending time thinking about how can I prevent 3- and 4-year-olds from being subjected to chemical weapons and nerve gas.”

He told other world leaders that they should “admit it” if they are looking for an excuse not to act against Mr. Assad.

But just moments later, he was reminded by his Swedish host how hard it will be to recruit partners, even in the face of dead children in the streets of Syria.

“This small country will always say, ‘Let’s put our hope in the United Nations,’” Mr. Reinfeldt said, echoing the sentiments of U.N. leaders who have cautioned against America going it alone.

While France and a few other nations appear willing to contribute, other key allies do not. Great Britain is unlikely to join in the effort after Parliament last week voted down military action.

When he arrives in St. Petersburg, Mr. Obama surely will face resistance from Russian President Vladimir Putin, whose nation has blocked U.N. resolutions that simply condemn the use of chemical weapons in the ongoing Syrian civil war. Russia’s position makes it all but certain that the U.N. Security Council will not authorize any action in Syria, although Mr. Putin, in an interview with The Associated Press, hinted that the door wasn’t entirely closed for action against Moscow’s longtime ally.

In the AP interview, the Russian leader warned against taking unilateral action in Syria but also said Russia “doesn’t exclude” supporting a U.N. resolution on punitive military strikes — if it is proved that Damascus used poison gas on its own people.

Mr. Obama’s talk of coalition building, analysts say, is important given the president’s statements on the issue. But in terms of military reality, the U.S. is more than able to carry out retribution against Mr. Assad’s government and doesn’t necessarily need help.

“From a military perspective, I don’t think it matters much given the mission we’ve heard about,” Mr. Friedman said.