We crossed this line when we established a "red line". You don't think Iran, North Korea, Russia, China are watching how we respond now very closely? Do you think Iran is going to take any "red line" seriously that we establish in regards to their nuclear program if we ignore our own red lines because we don't have the stomach to stick to them? If that is the case -- don't establish the red line to begin with.
Their are two options available to us. A world that is lead by the United States, or a world in which the United States is a follower. Which is it that we would prefer? The Boston bombing, and other terror attacks, are but a blip on the radar compared to what a nuclear armed Iran will mean for the Middle East. They are blip on the radar in regards to a rising China, or a Russia that is reasserting itself. These things matter, and sticking our head in the sand and ignoring it doesn't make it go away -- it just makes it worse. There is no retreating to our borders at this point -- that ship sailed decades ago.
The President is not a fool. I keep hearing this narrative, but the simple fact is he is a brilliant campaigner, who has put our party on the defensive for years and easily won reelection in a horrible economic environment.
As for compounding mistakes in Syria -- it depends on what "intervention" we do. No one says we have to follow the Iraq model -- but the simple fact is that our own intelligence (and others) believe Assad used these weapons on a small scale. I know the White House is terrified of an Iraq type mistake on this, but at some point you have to accept that the intelligence (however flawed) is all you have, and you have to make a decision. There are many other options available to "intervene" without a large presence.
If it is as you claim (which I don't believe it is), then it is of the utmost importance to regain credibility -- by doing what we say we are going to do. Credibility is the most important thing in international relations.