And of course we all know midevil peasants from across Europe thought just like neo-european youth of today. :rofl:
Seriously, Kings know it means a great loss of personal liberty for them, and they are the ones making the decisions and telling the uneducated masses what to think about those evil guys across the border. Europe ALWAYS fought with itself, until outside forces forced them to be peaceful and economics basically required them to form an economic union. Just think of all the crap that went on between Italian city-states durring this period! Never mind larger countries.
Almost never in Human history does our race ever do something just because it's logical. It almost always takes an irresistable force pushing us towards progress. The Roman empire didn't come to be because the Romans told everyone how great it would be to cooperate. They beat everyone into submission, and when their army started to fail people could have decided it was better to remain an empire, but no. Self interest and petty disputes overruled logic, and Europe was plunged into chaos and war.
This continued until finally WW II saw virtually all of Europe broken economically and militarily, and the outside military presence of the US and USSR enforced peace. With large economic rivals, it's now good for Europe to stick together, but if there is no rival, no outside force to unite against, Europe will fracture again. I'd bet the US would fracture as well without a powerblock to view as a rival. Oh wait... civil war, already happened once. With no outside force to rally against, internal feuds run amok.