In my fantasy story, there is a very important flashback: centuries back, there were 3 humanoid races - always in war with each other. The third race is more evil, more powerful(both physically & technically) than the other two. The first and second race formed an alliance and they finally defeated the third race. A few members of the third race escaped to another planet (they have high-tech), all others of the third race were killed.
In the present day, the remaining countries of the first and second race developed a lot on their own means and the third race is returning to the planet to avenge the defeat from centuries ago.
I hope it is possible that such a highly developed country is defeated by two other countries which are very poorly developed, for example if one of the races has ability to control nature - can make earthquake, tsunami etc.
My question is why should the three countries fall in war in the first place? I need a strong reason for keeping them in war and which must be strong enough to make the refugees of third race to return after centuries to pay the debt.