Many governments are dictatorships or autocratic. Some are democratic. I dont think it matters. You have the same responsibility to civilians regardless of the form of government.
> carte blanche to raze the city
Generally not. But you can certainly destroy buildings used for military purposes. Where this conflict falls on this scale is pretty unclear thus far.
> eliminate its civilian population
Definitely not. Israel has not done this.
> displace ... its civilian population?
Maybe. There are certain circumstances under the geneva convention where this is allowed, and it seems likely the criteria was met in this conflict. You of course have to let them back once the fighting has subsided.