2. Assad will be emboldened, too: Proponents of military action maintain that failure to use force will strengthen the regime and persuade Assad that he can act with impunity.
This is a stronger argument. There's no doubt that Assad will be emboldened and much of the opposition demoralized if the Obama administration fails to act. But -- putting aside for a moment the question of how destructive a U.S. strike might be -- would failure to act raise the odds significantly that Assad would deploy chemical weapons again? Alternatively, could striking really guarantee Syria would never use them again?
Cleary, we cannot be sure. Those in favor of military action, however, seem all but convinced that, although striking cannot ensure a stop to chemical weapons attacks, not acting will virtually guarantee that Assad will use such weapons more routinely. The administration believes this is one of its most compelling arguments -- that if the international community doesn't act, it will establish a horrible new norm whereby dictators have a green light to use chemical weapons -- but it is an extremely difficult proposition to prove. And there's more than a little hypocrisy on the part of the United States given its own acquiescence in -- even support for -- Saddam Hussein's use of chemical weapons against the Iranians.
I take and accept the general point that, while you can't stop war, you want to make it a little less barbarous and that enforcing international norms against the use of chemical weapons would help do that. But would it deter Assad? Maybe for now. But Assad and his murderous cohort are fighting for their survival, and if pressed, they will deploy chemical weapons again and again. Given the virtual certainty that the Syrian conflict will persist with many more regime and rebel ups and downs, a "one and we're done" set of strikes almost certainly cannot dissuade Assad from using chemical weapons again. Nor is it a foregone conclusion that a limited strike will forestall Dictator X or Y from using chemicals in the future, particularly if the U.S. action doesn't truly hurt the Assad regime.
3. Israel will be weakened: American supporters of military action against Syria -- and many Israelis, too -- take the position that failure to act will weaken the United States and thereby undermine Israel's security as well. This is presumably why AIPAC has been so active in supporting the administration's case on Capitol Hill.
There is some logic to the notion that when the United States appears ineffective and weak in the region, so does Israel. And that deterring bad guys in the Middle East is a critical component of both countries' policies.
But Israel is hardly a potted plant. It has demonstrated a consistent capacity to act even when the United States won't (see Iraq, 1981; Syria 2007, 2013). By now it should be pretty clear that, when it comes to security matters, the Israelis are pretty good at attending to their own business. They don't have a stake in seeing Assad or the opposition triumph in a definitive way, and they will continue to enforce their own red lines -- e.g., maintaining quiet in the Golan Heights and blocking weapons transfers to Hezbollah and other unfriendlies -- while the Syrian civil war continues. So I don't find the argument that U.S. inaction on Syria fundamentally weakens Israel all that compelling.
It is, however, a stunning demonstration of Israel's centrality in congressional debate that critics of U.S. military action contend the country might be harmed by a strike on Syria and that supporters argue a failure to strike would hurt its interests.