Some people are asking why we would strike Assad's forces in response to him attacking civilians whom the administration doesn't seem to want to accept as refugees. It is important to understand the bigger picture foreign policy implications.
Beyond any moral or ethical feelings connected to what happened, or to US refugee policy, are several responses based upon strategic concerns.
One answer is that the primary reason to respond to this sort of action by Assad is to slow the flow of refugees out of the country. The primary purpose of chemical weapons is to cause people to flee en masse. Assad's use of these weapons is precisely to cause the flight of large numbers of civilians, to make them seek refugee elsewhere. For every person killed in such an attack, a high multiple will flee in fear of the next one and the effect is exponential. A second use, third, and so on will create a massive flow of refugees. So stopping these attacks will also slow the flow of refugees out of the country.
Secondarily, as I have noted before, the easiest way for terrorists to acquire WMDs, in this case weaponized Sarin gas, is to have it drop into their midst and fail to detonate. Those munitions could be used to create a horrific death total in a confined space anywhere in the world. So there is a grave threat to our own security and to that of our allies that comes from Assad's use of such weapons against people who seek to do harm to us. We don't want such weapons literally falling into the hands of the Islamic State or Al Qaeda.
A third answer is that this was an excellent opportunity to take a relatively modest action that will establish some credibility to enforce diplomacy with the threat of military action. When dealing with those threatening the United States, its allies, and its interests, fear that the United States will carry out threats to use its military capability to respond is essential to the functioning of diplomacy. It is best that credibility be establish in a way that does not result in either mass casualties or the significant escalation of an already problematic conflict. More significant action may well be required to do this effectively, but this action could be seen as a start.
A fourth answer is that this puts significant pressure on Russia to reign in both Assad and Iran, while also putting pressure on China to address threats coming from North Korea as well as putting North Korea itself on notice that a US military response is now more likely than it once was.