"The nationalist not only does not disapprove of atrocities committed by his own side, but he has a remarkable capacity for not even hearing about them." -- George Orwell
A huge portion of my countrymen are living proof of this.
Eric Blair was making something of a generalization, but overall, the principle is historically true enough. The corollary is also true: sometimes a man can see nothing but evil in a nation, no matter what the evidence. At the core, it's a tribal thing, and a tribe can be a nation, a race, an ideology, or a religion. It can take some courage to stand back and be objective about the group one identifies with.