In the article Kings, Jacks and Jokers | How the nationalist lie is robbing Europe of its cards Sophie in ’t Veld explains that the EU is weak because of nationalism:

How come a Union that is bigger than the US, highly developed, with a healthy and highly skilled population, is the weaker party in the negotiations?
The answer lies in the fact that national governments leaders have deliberately obstructed further European integration in virtually every policy area for the past two decades. Their jealousy of a strong Europe carries a hefty price tag.