Or religion... In some ways it might be healthy for small communities. But can be rather destructive too.
I often lump religion and nationalism together, they're very much alike
It seems a recurring theme in our world that any large centralization of power lends to disastrous results. The crusades did not happen because one or two Christians got together and decided they'd try to take over the holy lands; the religion had a huge following and all had high respects for their various leaders; one Pope decided he needed to conquer and take control over the holy lands, and had the influence to control his followers through manipulation. The incredible amount of power religion had during this time is unreal, but the blinding patriotism of today is eerily reminiscent; it's like being trapped in this little mental box where your leader cannot be wrong under any circumstance; even if God says "No killing please", the Christians of this time found a way to ignore this, and even if good conscious says "No killing please", patriots find a way to ignore this. The "greater calling" always beats one's own humanity, it seems.