It is very easy.
The West made good (for themselves) terrorists to topple Syrian government cause Syria was a powerful anti Israeli country and had stood up against Zionists.
http://www.theguardian.com/commentisfree/2012/jan/17/syrians-support-assad-western-propaganda
http://www.voltairenet.org/article178779.html
https://en.wikipedia.org/wiki/Syrian_presidential_election,_2014
You can also find more sources on internet.
Syria is trying to clean these saudi, turkish, west, rabid dogs and also keep Syria indecomposable.
These rats such as ISIS, al Nusra and Ahrar al Sham etc... are several times more savage than taliban. Syrian army can not step back.
If Syrian army step back then lifes of millions Syrians would be in real danger and these west animals will kill each other and Syrian people.
Moreover the shameless american, turkish, saudi regimes are still supporting these dogs.
The problem is west made goons and Not legal Syrian government.