How does America justify bombing Syria?

How does America justify bombing Syria when Syria has remained neutral throughout the conflict, even taking in a few million Iraqi refugees? Granted, there were claims that Syria "allowed" militants to travel into Iraq, but at the same time Iraqi officials stated that they're wasn't much of a problem anymore with militants coming in from Iran and Syria. Besides, in America, illegal immigrants cross over the border in mes every day- how can America, a country with many times the resources and manpower of the much small Syria- attack Syria for something they can't do themselves? And why did it have to be civilians? Surely a few guys, a woman, and four children weren't a threat to US or Iraqi security...