There are nine main groups fighting in Syria, each with thousands of soldiers: 
1)The Islamic State of Iraq and the Levant (ISIL/ISIS)
2) Al-Nusra Front
3)Free Syrian Army
4)Ahrar ash-Sham
5)Asala wa-al-Tanmiya
6)Sham Legion
7)Ajnad al-Sham Islamic Union
8)Jaysh al-Islam
9)Jaish al-Fatah
Then throw in Syria, Turkey, the Kurds, Hezbollah, Iran, Iraq and, Russia, who provide money and/or logistical support.
It is frightenly complicated.  Obama, a Harvad MBA, a practitioner of critical thinking through the use of reason and logic, wisely decided to stay out of the quagmire.
The Game Show Host president brags that he doesn't read books or reports. He is adept at one thing: emotional responses to visual  or auditory signals from FOX TV. Ergo the missile strike.
Ahh, I think I will go with Obama's cautious approach any day, thanks.