The EU has recently enlarged to become a global player. This gave the EU some totally new traditions from now on. I'd like to gain some conceptual understanding about a few EU rules on this.

1. Why did the EU force some of the new EU members such as Slovakia and Hungary to re-accept their national borders? These borders are known to widely deviate from ethnic boundaries, locking Slovaks out into the Ukraine and Hungarians out into Slovakia.

2. The South-East European borders are defined by 20th century "peace" dictates from external super-powers, for the purpose of maintaining deep ethnic tensions. What interest does the EU have in maintaining this status quo?

3. How would the EU react if an EU ethnicity wanted to beak from its host country and re-join its mother country at the other side of the border, changing the border?

Thanks.