A)
Civil war breaks out all over Europe somewhere between the 80s and the 90s - maybe all at once, maybe one country after another, with few countries spared and different degrees of violence.
In some places, the people decide to split up themselves. In other countries, artificial borders are drawn by peacekeeping missions of the UN, the US, neighbouring countries, or the likes. Also, sometimes borders are changed according to voting results after the fighting.
B)
The EU becomes much stronger and decides that it doesn't need strong nations within. Most people agree, as they happens to identify with the EU, not so much with their homelands. So the big states are dissolved, the borders of the small states are corrected, smaller states are united with other states. Similar to the reordering of Germany under the fascists, but without the dictatorial elements.