The President’s speech to the nation Wednesday can be reduced to two essential points:

1. ISIS does not pose a present threat to the United States, but it does pose a potential long-term threat.   We must therefore act against it.
2. But that action should be limited: we are not committing, and do not intend to commit, significant ground forces to this effort.

I agree with both of these points, but they do involve a basic problem and a contradiction.

ISIS poses no near-term threat to the United States because it doesn’t have the international presence, with cells in multiple countries, that Al Qaeda did pre-9/11. But it does pose a long-term threat because, given control of a substantial expanse of territory, ISIS’s mini-caliphate would very likely serve as a safe haven and base of operations for jihadists the world over. I don’t think there is any reason to doubt that there are thousands of such people in the world whose cherished goals include the killing of large numbers of infidels, especially infidel Americans.

There are a lot of reasons not to commit ground troops to this effort. Most obviously: war is war and Americans will get killed. Besides, there can be no certainty that such a war would do more good than harm. It could well further inflame anti-American sentiment in the Muslim world and thus strengthen ISIS’s recruiting efforts. (It is, indeed, ironic, that it was US intervention in the first place that sparked the birth of Al Qaeda in Iraq and ultimately its offshoot, ISIS.) The ground campaign against ISIS must be carried on by Muslims who have the most direct and pressing interest in containing and defeating it.

The problem with Obama’s strategy, which centers on assembling a broad regional coalition to fight ISIS and supporting their efforts with UA air power and “advisers” on the ground, is that it just may not work.   It’s still not clear that an essential ingredient of that coalition—a credible national unity government in Baghdad that can enlist Iraqi Sunnis in the fight against ISIS—is going to happen. Nor has Obama explained how the idea of arming that elusive configuration, the Syrian “moderate” opposition, is less fantastic now than it was when he pronounced it a “fantasy” a few weeks ago. And other governments in the region are showing only tepid support, at best, for the US effort.

Realistically, then, Obama’s strategy doesn’t have great prospects for success. That brings me to the contradiction I mentioned earlier: if, indeed, there is a real possibility of failure of the president’s proposed strategy, and if ISIS represents a genuine long-term threat, wouldn’t we have to consider sending in ground troops?

The only sensible answer to that question is an answer that Obama dare not pronounce explicitly: while an ISIS state would indeed be a bad thing for the US and for the world, it would not be so bad as to justify the loss of blood and treasure in a ground war in the Mideast. An ISIS mini-caliphate would be bad but not catastrophic because once ISIS establishes a territorial domain, it becomes vulnerable in ways that stateless actors like Al Qaeda are not: it would have an address. It would have something to defend. It would, in short, be deterrable. If deterrence were to fail—if ISIS were in fact to become a launching pad for terrorism against the West—then and only then would there be justification for a full-scale ground assault.

Ah, but isn’t an ounce of prevention worth a pound of cure? Shouldn’t we just go in there and finish ISIS off now? No, because a ground war now wouldn’t be an ounce; it would be several pounds, and at this point we can’t be sure that it would ever truly be necessary. If the mere existence of an ISIS caliphate wouldn’t be justification for full-scale war, then certainly the prevention of the caliphate cannot be justification for such a war. But the prospect of a caliphate is bad enough to justify more limited action now.

Obama may well be thinking along these lines, but he can’t say so because he would be crucified by the political opposition at home.   An explicit admission that it might eventually be necessary to deter, rather than simply destroy ISIS would be fodder for the Republicans—a shameful admission of weakness. The difference between bad and catastrophic is a nuance. Unlike Obama, our Republicans don’t do nuance very well; neither does the broader American public.

So, Obama is right to keep the US engagement limited. (Not limited enough, in my view. His prospective intervention in Syria is a big mistake: more on that in a bit.) Unfortunately, as numerous critics have pointed out, once you get involved militarily, the logic of escalation easily takes hold as soon as success proves elusive.  David Corn of Mother Jones has aptly described Obama’s challenge—“how to unleash the dogs of war without having them run wild. “

If air strikes pounding suspected ISIS targets in Iraq don’t do the trick, is Obama obligated to bomb in Syria? If bombing in Syria doesn’t turn the tide, does the United States have to become more involved in the civil war there? If US trainers don’t sufficiently help Iraqi troops battling ISIS, does the president resist calls for introducing US special forces into the fight? If an Iraqi unity government cannot function, does the United States and other coalition members wage the fight against ISIS on their own? If the current crisis yields a wider Sunni-Shiite conflict, what the hell does the United States do?

Obama’s intentions are clear: he doesn’t want to return to full-scale US military involvement in Iraq. But now that he has committed the United States to renewed military action there, where’s the line?”

The line needs to be drawn by Congress. Congress should specify what the president can and cannot do. And, if the president at some point deems it necessary to cross that line, he will need to go back to Congress for authorization.   Congress needs to tie the president’s hands even if it later chooses to loosen the bonds. As Keven Drum has put it, only that way can we hope to prevent “a slow, stealthy escalation that flies under the radar of public opinion.“

Congress is not likely to do its job. At this writing, it seems that all Congress is about to do is authorize the training of the “moderate” Syrian opposition—a course that effectively commits this country to long-term involvement in the Syrian civil war, in blatant violation of international law.   We would do better to face the reality that after Iraq’s, Assad’s regime is the most vulnerable to ISIS and is therefore a natural ally with whom we should be working. If we could work with Joseph Stalin during World War II we should be able to work with Assad. But we won’t.

Obama faces a very tough situation. It’s impossible to do nothing about ISIS, but it is very hard to do something without eventually doing too much. I’m not optimistic.