They really have been studying human civilization for years. Finally they decided it was time to take action. One day, the sky became dark in the middle of the day, it was no cloud, it was a huge spacecraft. millions of rays shot down on earth, they sucked up every single weapon on earth, the database – hard and soft and the men who created them.
Only the men returned, unharmed but the slightest knowledge of weapon making erased from their mind, as if it was never there.
Giving humanity another chance, after all, it was going to take quite sometime to build from scratch.