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A cardinal rule in foreign policy is this: if your enemies are busy killing each other, don’t step in and try to stop them.  Yet, that’s exactly what we would be doing if we train, arm or provide military assistance to the Syrian rebels.

I’ve just returned from a week in Israel. While I was there I travelled to the Syrian, Lebanese, Palestinian West Bank and Jordanian borders.

I met with Israeli officials, journalists and military officers. Although they are reluctant to say so publicly, there seems little doubt in their minds that the strongest rebel groups in Syria are radical Sunni Islamists.  There may be other rebel groups that are secular and pro-Western, but they are weak, unorganized and unlikely to prevail without massive outside military assistance.  Those rebels might have been stronger today if we had helped them two years ago, but at this point they stand little chance of defeating either Assad or the other rebel militias.

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In other words, the Syrian civil war is now down to a fight between the mass murdering, chemical-weapons-using Assad government, and the Al Qaeda-affiliated, radical extremists.

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We don’t want either of them to win. But Iran, Russia, Lebanon, Al Qaeda, and some Gulf states all do. And they’ve lined up on both sides of the battlefield providing fighters, arms and aid.

The Syrian civil war has become a proxy battle between Sunnis and Shiites, and serves as a giant sponge soaking up militants from as far away as Europe and South Asia.

It is a tragedy of epic proportions.

But Americans don’t like to stand by while innocent people are killed and watch a human disaster unfold.  It goes against our very fiber. We feel compelled to DO something.

Arming the pro-Western rebels in Syria would give them a major assist, but it is unlikely to change the final outcome.

It also carries the risk that whatever arms we give them now could be seized by Al Qaeda rebel forces and used against us or our allies down the road. And, while establishing a no-fly zone might be a setback for Assad’s forces, the major beneficiaries will be the Al Qaeda-affiliated rebels.

Clearly, establishing a pro-Western, anti-Iranian, anti-Russian, secular post-Assad Syrian government would be a major gain for us and that of our allies. But that is an option we no longer have.

The cold harsh reality is that Syria is already lost.

So, is there anything we can do that advances our interests and has a reasonable chance of success?  Yes.

- We can provide humanitarian assistance to refugees who are living in refugee camps throughout the region.

- We can allow some Syrian refugees to emigrate to the US, provided they are fully vetted.

- And we can shore up our allies in the region, especially Jordan, which is most at risk.

There are officially half a million Syrian refugees in Jordan, some think there could be a million refugees before long.  Jordan is a poor country that is running out of water, natural gas, and food. A million refugees flooding into Jordan is like some 50 million refugees showing up on America’s shores.

If Jordan is to remain stable and independent it needs assistance now. If Jordan descends into economic and political chaos, it risks igniting the Palestinians and drawing the entire region into a widening conflict.

- Finally, we can make sure we know where Assad’s chemical weapons are stashed and work with our allies in the region to seize or destroy them before they fall into the hands of forces that would use them against us. -- We failed to secure Qaddafi’s weapons caches during the Libyan war.  Today they are being used in conflicts from North Africa to the Gaza strip.  We cannot make that same mistake with Syria’s chemical weapons.

President Obama laid down a red line stating what he would do were Assad to use chemical weapons. We now know he has.

Some argue that is reason enough for the U.S. to intervene in Syria; that our credibility is on the line. But President Obama is fond of red lines and making threats – to Iran, to North Korea, to Syria. His credibility is already damaged. It will be much more so if we aid and arm Syria’s rebels and our efforts fail.

In fairness to them, Obama nor the Republican interventionists suggest that we should put U.S. boots on the ground in Syria.  But in fairness to the American people, enforcing a no-fly zone or training rebel forces are military operations, despite what we call them.-- It’s like calling taxes “revenue enhancements.”

One of the lessons of Vietnam, which we failed to heed in the Iraq war and the Afghanistan surge, is that before you commit U.S. military forces to aid or assist, it is essential to know what you want them to achieve.

Without a clearly defined mission, it is impossible to know what kind and how much assistance to give, or predict how long the commitment will be.

Once we send military aid, or American troops to train and fight or set up a no-fly zone,  it is crucial that we match resources to mission and means to ends.

We must also be prepared to adjust both as necessary, since the enemy will inevitably adapt and the situation will evolve.

We should have a sense of what constitutes victory, and what is the likely outcome once the shooting stops.

Finally, we should be upfront with the American people about the costs of our involvement, both human and material.

If all those sound like limiting factors, they are. It is all too easy for our political leaders to intervene in conflicts. That is why we should only commit American forces to fight if it is in America’s vital and essential national self-interest, and if we are prepared to do whatever necessary to prevail. In that case, losing is not an option.

On the other hand, the only thing worse than not getting involved in a fight we feel is just, but not vital, is getting involved and then losing, or despite our best intentions, making things worse.