This article encapsulates the reason why it is necessary for the United States to intervene in the Syrian civil war. Simply put, America's street cred is on the line and if The Most Powerful Man In The World (aka, the President of the United States) draws a red line and fails to act decisively, that completely destroys the credibility of the United States in the realm of foreign policy.
Credibility is what keeps the world turning. Credibility in money allows it to be used as a store of exchange. Credibility in the law (and the consequences of breaking it) is what prevents people from behaving like animals. If you say you're going to kill the king, you better kill the king.
A lot of countries have staked their security on the implicit and explicit promises made by the United States government. The only thing preventing nuclear armaments from popping up in Japan and Korea is the American nuclear umbrella. Saudi Arabia, the most powerful Arab state, follows our lead because they trust us to do the dirty work in the region. Germany has resisted real rearmament because of NATO.
The United States has created a global framework in which the US has a military that is second to none, whose military expenditures engulf the rest of the world's, despite the fact that the US only accounts for 20% of the world's economic output. Everybody else has grown accustomed to this fact. And they are content to let us lead world affairs so long as we remain both powerful and credible.
That latter trait has become increasingly suspect in other capitals. US support and reassurance has prevented Israel from unilaterally acting against Iran and its nuclear weapons program. It has also kept a lid on the situation in the Korean peninsula and in Taiwan. It has deterred Russia from rearmament and reclaiming its "near abroad" as a formal sphere of influence. This has reduced the global share of military expenditures to extreme lows, which has then been reinvested in the global economy. So long as people are content with the US having the guns, they will churn out more and more butter.
That system that we've created is being pressured by the likes of Syria. If Syria can use chemical weapons and the US doesn't act on a clear transgression of international law, Saudi Arabia and Israel can't rely on us when it comes to stemming Iran's nuclear ambitions. South Korea can't trust us to act when it comes to North Korea's constant provocations. Japan and the Philippines can't trust us to contain growing Chinese ambition in the Pacific. US inaction in Syria will be a catalyst in creating a new arms race. And that ultimately endangers national security.
President Obama is stuck between a rock and a hard place. Failure to act means a very substantial loss of face both in domestic politics and internationally. Any military action will be controversial and likely unpopular. Actions from the White House and Foggy Bottom suggest that the Administration wants to offload as much responsibility as it can to Congress and our allies. But there is no decisive action without US involvement.
Simply put, the President needs to nut up against Assad. Erode his military assets via cruise missiles and keep it up until his chemical weapons infrastructure is completely destroyed. Then turn the fleet back to Italy and call it a day. It appears that a limited engagement appears to be all that the Administration is gunning for. But stalling won't make it any more palatable of a choice.