A CIA spokesman says a new intelligence assessment estimates that the Islamic State group can muster between 20,000 and 31,500 fighters across Iraq and Syria, up from a previous figure of 10,000.

The new assessment is based on a review of intelligence reports from May to August. It is larger than the 20,000 figure being used by many outside experts.

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Arab nations vowed on Thursday to “do their share” to confront and ultimately destroy the Sunni extremist group known as the Islamic State in Iraq and Syria. The promise came after the nations’ foreign ministers met here behind closed doors with Secretary of State John Kerry.

A joint communiqué issued by the United States and 10 Arab states endorsed a broad strategy to stop the flow of volunteers to ISIS, curtail its financing and provide aid to communities that had been “brutalized” by the militants…

None of the Arab participants said precisely what they would do, and it remained unclear whether any would join the United States in mounting the airstrikes.

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Syria has “no reservations” about U.S. airstrikes against ISIS and wants to team up with Washington to tackle the militants, the country’s deputy foreign minister told NBC News.

Faisal Mekdad called Syria’s President Bashar al-Assad “a natural ally” for the U.S. in its battle against ISIS, saying in an exclusive interview that both countries are “fighting the same enemy” and should be working together — not antagonizing each other.

“When it comes to terrorism, we should forget our differences… and forget all about the past,” Mekdad said. “It takes two to tango…We are ready to talk.”

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In Iraq, dissolved elements of the army will have to regroup and fight with conviction. Political leaders will have to reach compromises on the allocation of power and money in ways that have eluded them for years. Disenfranchised Sunni tribesmen will have to muster the will to join the government’s battle. European and Arab allies will have to hang together, Washington will have to tolerate the resurgence of Iranian-backed Shiite militias it once fought, and U.S. commanders will have to orchestrate an air war without ground-level guidance from American combat forces.

“Harder than anything we’ve tried to do thus far in Iraq or Afghanistan” is how one U.S. general involved in war planning described the challenges ahead on one side of the border that splits the so-called Islamic State.

But defeating the group in neighboring Syria will be even more difficult, according to U.S. military and diplomatic officials. The strategy imagines weakening the Islamic State without indirectly strengthening the ruthless government led by Bashar al-Assad or a rival network of al-Qaeda affiliated rebels — while simultaneously trying to build up a moderate Syrian opposition.

All that “makes Iraq seem easy,” the general said, speaking on the condition of anonymity to share views on policy. “This is the most complex problem we’ve faced since 9/11. We don’t have a precedent for this.”

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So, the cause is just, and Obama’s plan sounds reasonable, even nuanced. What could go wrong? Well, as anyone who’s studied the region (and the cavalier predictions made, time and again, by Westerners who go to war there), everything.

Obama made very clear that this battle requires active participation by the Saudis, Turks, and Europeans. But the roles and missions haven’t yet been outlined; the commitments aren’t quite carved in concrete. The plan has a chance of succeeding in Iraq because the new government, formed by Haider al-Abadi, seems inclusive, embraced by Sunnis and Shiites, for the moment—but it could fall apart with the bombing of a single mosque or a marketplace, and then what? Will it look like the Americans are advising and bombing on behalf of a Shiite regime? Will the other Sunni nations back away, fearing the association?

As for Syria, the endgame is unclear. If the Free Syrian Army can’t get its act together, despite all efforts, will Obama step back from that terrain and focus again on Iraq—or will he be tempted to escalate and take on more of the fight alone from the air? Obama is allergic to “mission creep” (and that’s good), but he has said that this war will go on for a while; his advisers were recently quoted as saying at least three years. Where will the next president take the fight? To draw a Vietnam analogy (which, granted, should not be stretched too far), will he or she be Lyndon B. Johnson to Obama’s John F. Kennedy? (JFK sent only advisers to Vietnam, refusing to deploy combat forces.)

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“A tricky situation is going to be how the U.S. will ensure that it is only ISIS targets that are hit, rather than civilians and others,” said Haras Rafiq of the Quilliam Foundation, a London-based think tank.

The U.S. would most likely target supply lines and heavy weapons to erode the group’s power, rather than wiping it out, said Eliot Higgins, who analyzes weapons used in Syria. “It’s really about targeting the stuff they can’t hide easily.”

Higgins said the U.S. was unlikely to attack militants in cities and towns, where airstrikes could cause civilian deaths and give the group a propaganda victory.

It also would also be difficult to target them if they were attacking government assets, such as military bases, because the U.S. would “end up being air support for the Syrian government,” he said.

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For Iraq, Obama pledged more support for Kurdish militiamen, pro-government fighters and a new National Guard-style force that he suggested would be primarily Sunni. Each recipient of the U.S. largesse is problematic.

The Kurds have used Iraq’s crisis to expand their northern autonomous region and make big oil plays without Baghdad’s consent; there’s no guarantee that any training they receive would go toward keeping Iraq intact and sovereign. At the moment, the pro-government forces are mainly Iranian-backed Shiite militias. And it’s hard to imagine Shiite leaders swallowing the creation of a Sunni armed group that one day could challenge them for sectarian supremacy.

In Syria, it’s still unclear which “opposition” Obama means. He made no mention of the Free Syrian Army or the Supreme Military Command, two previous groups the U.S. government said it was equipping. Analysts say there’s no quick way to recruit, vet and train a rebel force in Syria; any such endeavor would take years and is no guarantee of success, as the United States saw with the collapse of its trainees in Iraq. And the U.S. won’t work with the two existing forces – the Syrian army and Kurdish rebels in the north – that could challenge the Islamic State.

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One major concern about American intervention in the Iraqi mess is that, by joining the fight against the Sunnis in the Islamic State, the United States would effectively go to war on behalf of the Shiite side of the sectarian divide. Everyone agrees that would be a disaster. “This cannot be the United States being the air force for Shia militias or a Shia-on-Sunni Arab fight,” retired Gen. David Petraeus said over the summer. “It has to be a fight of all of Iraq against extremists, who do happen to be Sunni Arabs.”

Despite being an outcome that all Americans want to avoid, that could be exactly what happens. “We’re already seeing reports where U.S. strikes against ISIS are having the effect of bailing out Iranian-backed Shiite terrorist groups,” said Republican Rep. Ron DeSantis, who served in Iraq in 2007 and 2008. “When I was in Iraq, once al Qaeda in Iraq was defeated, the main military front was taking on the Mahdi Army, a terrorist organization. Now, you could be in a situation where we are essentially serving as an air force for Shiite terror groups.”

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The job of the commander-in-chief is to defend our nation, not to promise the most expedient tactics. And while I hope and pray that we can, in fact, “destroy” the Islamic State without “American combat troops fighting on foreign soil,” it is premature to make such a promise. What happens if our allies on the ground can’t advance into the Sunni Triangle? What if the Islamic State, despite repeated air attacks, proves every bit as resilient as Hamas — which has weathered years of air strikes yet still maintains its iron grip on Gaza? Then which promise controls? The pledge to destroy or the pledge not to engage in ground combat?

When I heard the president speak of a long air campaign, what I heard was a pledge to kick the can down the road, to “do something” until his successor relieves him of the burden of fighting jihad. In so doing, he’s taking an enormous gamble with the safety and security of the American people. Let’s recall that the Islamic State controls a nation-sized land mass and disperses among the civilian population. Its ranks include hundreds of Americans, Britons, and other holders of Western passports. And let’s also recall that jihadists thrive when they’re seen as surviving and enduring American attacks…

Last night the president made promises. But the most important promise, to destroy our enemy, is the one he seems least willing to keep.

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This is the central irony of Obama’s speech—and, it must be said, of his approach. The caution that he has shown, the time that he has taken to reach a decision, are admirable and wise; the course of action that he has set out is, despite its increasing scope, narrowly targeted. (This is no war on terror or on radical Islam.) Even so, as he acknowledged last night, “we cannot do for Iraqis what they must do for themselves.” And there is, at this point, little to suggest that Iraqis can do much of anything for themselves but continue their slide into mutual mistrust and retributive violence. The security forces that Obama has now pledged to train, equip, and advise are seen, by many Sunnis, as a force of subjugation; Shiite militias, empowered by the previous Iraqi government and backed by Iran, have terrorized the population we intend to protect. The situation in Syria is less promising still. The anti-Assad rebels there have been unable to keep their weapons out of the hands of ISIS, which does raise the question: which side will we be arming?

In this sense, last night’s peroration—with its ode to American exceptionalism—was beside the point. Not because America isn’t terrific, which it is, or because our “technology companies and universities” aren’t “unmatched,” which they are, but because America’s success in this new and important mission will not depend, in the last analysis, on our values, our strength, or our can-do spirit. It will depend on partners who are at best unreliable and possibly incapable. If they falter, what becomes of the U.S. effort? That question was neither asked nor answered in the President’s speech, but there is always next year.

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The president may want to cherry-pick his enemy of the moment, but members of our would-be “coalition” — like key Gulf Allies or Jordan — are willing to participate only to the degree that the effort extends beyond IS to the broader threats that are of greater concern to them. If we drag our feet on this expanded target list (as we will), so will they.

More problematically, some possible members of this coalition (see: Qatar, Turkey) actively support some of these other groups. Combine that with the fact that our actions may actually help advance the interests of Iran, Shiites in Iraq, and Assad in Syria — all anathema to key members of the coalition — and you can’t help but conclude that holding this group together will be much more difficult than actually convening it…

The result is that once again the undoubtedly well-intentioned instincts of Barack Obama have run up against the harsh, complex realities of a Middle East in which no conflict has only two sides or a good outcome that doesn’t create new risks. It would be hard for the president to admit, but this is precisely the problem that confounded each of his predecessors during the past two decades. Each responded to the challenges differently and none could achieve the outcomes they sought.

Hence, there is only one conclusion one can draw from last night’s speech: What Obama began last night will be left to another president to finish. And it will continue to be a troubling constant in the life of a generation of Americans who have never known life without their countrymen engaged in military action in the Middle East.

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In plain English: We don’t really have a plan. We don’t have a definition of success. We see some evildoers and we’re going to whack them. They deserve it, don’t they?

And sure, ISIS does deserve it. The group is a nasty collection of slavers, rapists, thieves, throat-slitters, and all-around psychopaths. The trouble is: so are the people fighting ISIS, the regimes in Tehran and Damascus that will reap the benefits of the war the president just announced. They may be less irrational and unpredictable than ISIS. But if anything, America’s new unspoken allies in the anti-ISIS war actually represent a greater “challenge to international order” and a more significant “threat to America’s core interests” than the vicious characters the United States will soon drop bombs on.

The question before the nation is, “What is the benefit of this war to America and to Americans?”

That was the question the speech left unanswered. And the ominous suspicion left behind is that the question was unanswered because it is unanswerable—at least, not answerable in any terms likely to be acceptable to the people watching the speech and paying the taxes to finance the fight ahead.