Do you think that humans will ever be able to live together in harmony?

In my opinion, no. Somebody will always do something that prevents that from happening.

It would take aliens coming to Earth and showing us all how inferior we are, then maybe.

as ideal as that sounds no, at least not in our lifetime's humans are competitive by nature its a trait that helped us during times of survival, although it can be overcome with will power, it is difficult for (insert current earth population here) to be able to do it at once. it would be nice though.

The majority of the people can if they would just realize there are others that like to keep them divided for their own interests.

Yeah i agree, they seriously can just realize and hopefully stop.