Something sparked a memory of this short story I read years ago(over 10 I think) in a collection, not all by one author and I have been trying to find it since and haven't had any luck. I don't have the author or the titles. It's in the future and Earth governments are still separate and warring and aliens show up. They maybe say something about seeding Earth and waiting to see how humans develop and seem to be disappointed. Humans are frightened enough to have a summit of goverments to come to agreements regarding peace. The aliens say they didn't want peaceful people, they were planning for warriors and soldiers, they expected violence and survival of the fittest. They would have to destroy humans and start over.