Jerry Seib on Syria: So here’s the ultimate irony: The administration worries that lack of military action now will erode hopes for diplomatic action later.

Gerald Seib writes in The Wall Street Journal:

President Barack Obama goes on national television Tuesday night to make what has become a high-stakes and uphill argument for action against Syria - an argument that figures to divide into three broad categories.

First, he will offer a moral argument: As the world's pre-eminent power, the U.S. can't stand idly by while the Syrian regime uses chemical weapons against its own people.

He also will offer a national-security argument, one proponents of action complain he has failed to make coherently so far: America's own security is undermined if bullies on the international stage are allowed to amass and then use weapons of mass destruction.

But ultimately, the president's broadest and most controversial argument is likely to be the credibility-gap argument: American credibility will be undermined, not just across the Middle East but around the world, if it doesn’t follow through on threats to punish Syria, with potentially dire consequences down the road.

This is the ace-in-the-hole argument for presidents in this kind of precarious situation, pushing for action in the face of doubts about their plans. You may not be convinced that we need to act, the argument goes, but surely you can agree with me that the dangers the country and the world face will multiply if American might and conviction are no longer taken seriously.

In this case, the skeptics, including many in Congress, aren’t convinced. In their view, the argument for intervention in Syria’s civil war is sufficiently weak that other actors on the world stage will make a distinction between this mess and others where American interests are more obviously and directly at stake. And in those cases, they argue, U.S. resolve won’t be questioned.

“The conventional wisdom of everybody who favors a strike in this town … is that nobody will take him seriously on any other issue,” says Aaron David Miller, a former Middle East negotiator under presidents of both parties and now a vice president of the Woodrow Wilson Center. “I just don’t buy it.”

Still, it’s fair to ask where the credibility argument might hold water, if only because Mr. Obama now faces the very real prospect that he won’t get what he’s seeking from Congress. That would be a stunning outcome, raising deep questions not just about the president’s leadership and competence, but more broadly about whether the U.S. is retreating from a world leadership role.

In short, the credibility question may well prove more than hypothetical.

Mr. Obama doubtless will say credibility matters first and foremost in the case of Iran: If Washington declares that Syria can’t be allowed to use chemical weapons and then fails to act when it does, Iran won’t take seriously American declarations that the world can’t allow it to develop nuclear weapons.

But there are other, more subtle possibilities as well. Would American failure to act on Syria also make it more likely that Israel would decide to launch its own, unilateral strike against Iran’s nuclear facilities, because the Israelis will conclude they can no longer count on the U.S. in the face-off with Tehran?

It’s also possible that failure to act would further diminish American influence on the Egyptian military, which recently tossed out that country’s elected Muslim Brotherhood government. If the U.S. appears to be retreating from the front-line role it has played in the Middle East for the past four decades, Egypt’s new leaders may decide they have less incentive to heed American pleas that they temper their crackdown on Islamists in their streets.

There also is the danger that the already-strained effort to work out an understanding with Afghanistan’s government over security cooperation as American troops leave could become more difficult. How readily would Afghan President Hamid Karzai lean on cooperation with the U.S. if the U.S. isn’t seen as a reliable actor? Better, he may conclude, to strike the best deal with his Islamist foes.

And would Iraq also conclude it can’t rely on U.S. security guarantees and instead make a more overt cohabitation deal with Iran next door?

In the end, though, the biggest risk in the credibility game may well be to America’s Syria strategy itself. Though the administration has failed to convey this message particularly well, its underlying goal in launching a military strike would be to shake up the balance of power enough to push Syria and its Russian benefactors back toward a diplomatic solution.

Without more military pressure, it’s likely Syria and Russia won’t see much reason to negotiate anything. And the Syrian opposition, sensing it doesn’t have America at its back, might well see diplomacy as a trap that could bring about its demise.

So here’s the ultimate irony: The administration worries that lack of military action now will erode hopes for diplomatic action later.