Why might the problems caused by artificial boundaries continue after the europeans left?

As with all things artificial when it comes to the division of nations there is bound to be problems. After Europeans left the colonies were developing into new nations. But those nations were based not on any traditional boundaries but on those drawn by the European powers. This leads to fighting between the ethnic groups and tribes and there is no European military to stop the fighting. 

Answership building; new england was almost entirely mercantilist