How would you deal with "Nazi" Aliens?

I just "read" about this issue on another website: An alien race was discovered which had a "Nazi" like society with a goal of "world domination" and a desire to "exterminate" other races.<br><br>The leader of Earth (I presume the UN) said it was humanity's duty to exterminate them, but this raised questions of how this would fit in with the principles of refraining from war crimes.<br><br>This issue is compounded by the difficulty of communicating with the "Nazi" race as they only communicate in an "aggressive" tone.<br><br>I know this is probably a fairly common issue in worldbuilding (I'm basically a complete newbie to this field myself) but I'm wondering how this kind of issue would be resolved in practice?<br><br>On one hand you'd need to just "annihilate" the "Nazi" race (the website I read only mentioned conflict from one side, with all conflict being caused by the "Nazis") but on the other hand this is not morally acceptable from a social perspective and probably should be punished under international law.<br><br>How would this be handled?<br><br>Edit: I don't know if this is the right place to ask but I thought this was the best place to ask this question.