Syria upping the ante reportedly ready to deploy S-300 surface to air missiles to defend its air space

It's been reported Syria is ready to deploy the S-300 air defense system, a surface to air missile supplied by Russia giving Syria control of its territory w/ the capability to shoot down any intrusive warplane or missile w/out warning. That sure ups the ante. But Syria is a sovereign state & under int'l law it's been attacked illegally. Does it not have the right to further defend against any hostile force attacking it?