In setting I am working on, humans have been brought to the world as mind controlled slaves of a race traveling between dimensions. This master race travels between worlds and harvest the energy to power up its empire, which results in worlds destruction.
Prior to this, the world has been inhabited by elves, dwarfs, gnomes etc. In face of the invasion, those races fought the invaders. Ultimately, humans have been freed from their masters' influence, allied with other races and defeated the invaders.
Humanity is very expansionist race, so in world's present (few thousand years later), humanity dominated the world. Elves, dwarfs etc. are minorities living in small kingdoms or ghettos within human cities.
For typical human above ancient history is silly legends spread by filthy elves. Human's life is short in comparison to elves. For humanity, several generations passed, but many elves have grandparents who fought in one of these wars. This is one of the reasons for tension between the races.
How can I explain how the history has been forgotten by the humanity?
It seems that evil race that can control human minds is not something you should forgot about.
Edit To rephase the question to avoid Idea Generation : Are there real-world, historical cases that society has forgotten some of its crucial history? How has it happened (obviously it also had to be re-discovered later)? I am more interested in some kind of natural process rather than intentional propaganda targeted to send some events into oblivion.