With the likelihood that the House will vote not to authorize the use of military force in Syria it seems appropriate to consider the consequences that will flow from America's choice not to act.
In addition, one good way of thinking about the strength of the case for American military intervention is to consider what happens if that intervention doesn't occur.
The first order consequences are relatively clear:
(1) Assad will be free to use chemical weapons without fear of external consequences;
(2) Assad will use more chemical weapons;
(3) The United States will not incur the costs of intervening in Syria (probably a few billion dollars and a chance of a small number of casualties);
(4) The United States will not be morally responsible for directly killing some number of Syrian civilians;
(5) The United States will be morally responsible for failing to prevent the deaths of other Syrians; and
(6) The number of Syrians killed as a result of the Syrian civil war will probably increase if we don't intervene.
There are, of course, broader consequences to a decision of this magnitude:
Consequences for Syria
The consequences for Syria depend on whether Assad will choose to continue to use chemical weapons occasionally or use them more broadly once the threat of American military intervention is removed. Factors involved in this decision will include (i) the degree to which the Syrian hierarchy views chemical weapons with particular revulsion; (ii) the degree to which Iran views chemical weapons with particular revulsion; (iii) the degree to which the rebels threaten victory; and (iv) the degree to which Assad believes it to be in his interest to ethnically cleanse Syria.
If Assad uses chemical weapons more broadly, then the number of Syrians dying in the immediate future will increase and will include more noncombatants. The number fleeing the country will also increase, expanding refugee populations in surrounding states. How much of an advantage to Assad chemical weapons will actually be is something we can't know in advance.
Intervening decisively against Assad will save those lives that he otherwise would have taken, which is a far greater number than our intervention will cost. It seems likely that intervening against Assad will shorten the course of the Syrian civil war and lead to fewer Syrian deaths overall; but constructing reasonable scenarios where our intervention has the opposite effect is not that hard. So one effect of our not intervening will probably be more death and suffering in Syria, but the inability to quantify either the probably or the more weakens the argument for intervening.
Consequences for Lebanon, Jordan, and Turkey
The differential effects on the states adjacent to Syria of our failing to act are limited.
Refugee populations in Lebanon, Jordan and Turkey will continue to increase in any event. The increase will be larger if Assad uses chemical weapons more broadly. Relatively speaking, American intervention would probably increase the portion of refugees fleeing to Lebanon, but it is impossible to determine how the overall number of refugees would be altered if we intervened.
Consequences for Israel, Iran and the Gulf States
The utility of Syria to Iran has been severely compromised and will continue to be limited so long as the civil war continues. For Iran, Syria is now an expensive mess absorbing weapons, manpower and other military resources. I would suspect that eliminating Assad's regime would still hurt the Iranians more than allowing their effort to sustain him to continue to use up resources, but the ultimate answer could be the opposite. To the extent that neutralizing Syria as an Iranian ally is an American foreign policy goal, that is largely achieved so long as the civil war continues even without our military intervention and we still have the option of providing weapons to the rebels if doing so helps achieve that objective.
Whether Assad is removed or not is largely irrelevant to Israel. As long as Syria remains in the throes of a civil war, Syria and Hezbullah are not focused on threatening Israel. Probably the best longer term outcome for Israel is that Syria splits into several small, weak and mutually hostile states. Since it is unclear what the results of our intervening in Syria on the course of the civil war would be, it is unclear whether that intervention would benefit Israel. In any event, our failure to intervene wouldn't harm Israel.
For the Gulf States, especially Saudi Arabia, having America remove Assad would be a clear win, but American inaction imposes no immediate or medium term negative consequences on those states.
Consequences for the International Norm Against the Use of Chemical Weapons
Upholding the idea that chemical weapons use is unacceptable and will be met with such severe consequences that using chemical weapons is a losing proposition is the primary goal of and argument for American military intervention.
Because other Western nations, particularly Britain and France, are unwilling to act in concert with America and because the American public is so opposed to intervention, the actual idea that American intervention is likely to uphold is: (i) if America doesn't like you; (ii) you happen to be in an area where America has lots of military assets; and (iii) the President is willing to incur political costs to punish your use of chemical weapons, then using chemical weapons is unacceptable and will be met with such severe consequences that using chemical weapons is a losing proposition.
While there is some value to this, it is far less universal and far less likely to deter other dictators like Assad who might contemplate the use of chemical weapons, reducing the benefit we would hope to achieve by intervening and the collateral consequences of not doing so.
Consequences for America
Our credibility in situations where we have limited strategic interests may take a hit if we do not take military action in Syria. The immediate concern is that Iran and Israel will be less likely to believe American statements that an Iranian nuclear weapon is unacceptable and developing one may be met with American military action. However, it is unclear how much credibility this costs America. Further, in the particular case of Iranian nuclear weapons, Congress has demonstrated significant support of the American position, we do have a strategic interest in preventing Iran from acquiring nuclear weapons and the American public is supportive of military action. All of these factors would mitigate any loss of credibility which is, in any event, impossible to pin down with any degree of precision.
President Obama will, presumably, be more likely to consult with Congressional leaders before setting out future redlines. If he does this, of course, that will tend to remedy the loss of credibility we might otherwise suffer.
Affiliates of al-Qaeda will be less likely to get their hands on chemical weapons if we don't intervene. On the other hand, since Assad would retain his air force, the al-Qaeda affiliates will likely receive a larger number of "shoulder-fired" anti-aircraft missiles, which, though less of a concern than chemical weapons, are certainly cause for worry.
Thus it seems that the outcome for America from not intervening would be mixed and limited.
Overall Consequences of Not Intervening
To sum up, the broader consequences to America and its allies and the world as a whole of America failing to intervene in Syria are both very limited and not entirely negative, which is a large part of the reason the President is having so much trouble gaining Congressional and public support for this war.
The consequences for Syria of American inaction is likely to be more death and a longer civil war. But while intervention would likely reduce the length of Syria's civil war and the overall death and suffering of Syrians, pure humanitarian motives have rarely, if ever, been sufficient to motivate American military action.