The US used up its political capital by helping with Libya.
All this talk of the US and Europe only caring because of the oil seems to be ignoring the fact Libya has like 2% of the planets oil reserves.
Europe was concerned for the massive influx of immigrants, particularly in France and Italy. The US and Britain got involved because they have never really been big fans of Ghadaffi. In fact until Britain and France brought it up to the UN, the US was content to not take action at all.
Even if the US wanted to help in Syria, our populace would never go for it, it would be political suicide for the president or anyone in congress to even suggest it at this point. Europe could very well go it without us, and if you think they don’t have the military might to do so without the US you’re a fool…. It would just be more complicated without our help.