Now that Bashar al-Assad has (probably) crossed the Obama administration's "red line," U.S. officials say "all options are on the table." But you can be pretty sure that a direct attack on Syria isn't one of them. Despite the international condemnation that's already coming Assad's way, it's a near certainty that the United States will not commit to sending troops into Syria. So what can America do punish the regime?

Well, the first the thing the White House wants to do is conclusively establish that Assad is the one responsible. The only hard evidence that sarin nerve gas has been used on the Syrian people comes in the form of blood samples from the victims that tested positive for the deadly poison after the main attack in question, last month in Aleppo. Beyond that, American officials and their allies could likely never prove beyond reasonable doubt how the gas got there, when it happened, or who was responsible. There's no reason to doubt the blood samples are authentic, but it's been extremely difficult, for the two years and counting of this bloody conflict, for Western agents to gather intelligence from inside Syria. And what little evidence they do have would hardly hold up in court.

It might hold up in the court of public opinion, but the U.S. can only play the prosecutor—not the judge or jury. The next step is likely to take evidence to the United Nations, where Russia (and China) have blocked every effort to pass meaningful resolutions against Assad. But chemical weapons might be the tipping point that forces Vladimir Putin to abandon his old ally, or at least look the other way. (Especially after his tough talk about terrorism today.) When talking about "red lines," the U.S. isn't talking about what they find acceptable. It's about finding a crime that the rest of the world simply can't ignore.

If Russia and China can be convinced that Syria has finally gone too far, there could be a Security Council resolution. If that happens, the U.S. will probably look to follow the same model it used in Libya in 2011. The best case scenario is the declaration of a no-fly zone, to eventually be enforced by NATO. It's the classic "leading from behind" scenario where the U.S. mobilizes its allies, then let's them do the work. Britain and France have already talked about arming the rebels, but they probably won't do so without the force of the United States to legitimize their move. With American backing and Russia acceptance (if not support) those other efforts can go forward as well.

It's true the Syrian army and air forces are much more formidable than Mummar Qaddafi's military ever was, but there are other circumstances in the mix—namely:Israel, Lebanon, and Iraq. The use of chemical weapons is also a circumstance that didn't exist last time the Obama doctrine really went into effect, and it's one that might make it easier for both the world community and the American public to go along. It's worth noting that while Republican Congressmen are calling for leadership, they aren't calling for American troops to rush into Damascus. But they are onborad with idea of doing something.

No matter what happens, it isn't happening in the immediate future. The very phrase "red line" is carefully vague, and no one—including President Obama himself—has ever said what the specific consequences of crossing that red line would be. Today's letter from the White House makes it plain that the U.S. is leery about making the wrong call, and that the administration can't go to the U.N. with less than a bulletproof case, no matter what Chuck Hagel and John Kerry say. Even if the U.S. gets everything it can on what appears to be Assad's deadly gas, it won't be enough for the White House to believe it. The rest of the world does too.