If there were aliens to come explore Earth, what makes you think they will be here to conquer? Just human history? Well, we can't judge what aliens are going to do based on how we act, as they probably have developed in different conditions, and may not have even similar goals or nature as us. They may not even be able to comprehend war, for all we know. However, it is still possible that they could be worse than us, and just blow up the planet once they realize it's inhabited.
