Plenty of aliens came to visit Earth because they needed resources from planet earth. Some humans contacted the aliens to tell them that they (Aliens) could visit them and take what they needed in exchange for peace and after this they had to leave. It was not as easy as that though, before coming to an agreement they had a little war. This was kept a secret because the rest of the world couldn’t know about the existence of aliens, it would freak everyone out and even more if they knew that they came to earth. Many countries joined this war so that they could prevent chaos in the whole world. They ended up in peace so the aliens proceeded to leave our planet and not come back in a few years.