Aah........ Ok If I had put this in different words...

Colonization had developed a gateway between the two countries... and of course resources which one has in abundance n other is in scarce shall be distributed....anything...
And second reason is acceptability of each other ....

These factors are major reasons for the Migration...
It is not about the revenge....

The people who migrate do not do so for revenge.. for God sake.. it is necessity... and often also a desire to see something new....
Revenge or pay time are terms coined by people after seeing the localities not feeling comfortable with migrants anymore..... Jus for sake of satisfying there pseudo-ego may be...

Hard Work Doesn't Kill Anyone... But Why take Chances.