… is from Lord Acton’s 1862 essay “Nationality “:
The pursuit of a remote and ideal object, which captivates the imagination by its splendour and the reason by its simplicity, evokes an energy which would not be inspired by a rational, possible end, limited by many antagonistic claims, and confined to what is reasonable, practicable, and just. One excess or exaggeration is the corrective of the other, and error promotes truth, where the masses are concerned, by counterbalancing a contrary error.
UPDATE: Coincidentally, after posting the above quotation – one that I’ve long admired – I found this post from earlier today by Bryan Caplan over at EconLog . Indeed, I cannot resist offering these two paragraphs from Bryan’s post as an alternative quotation of the day:
There is however a less obvious, but far more important difference between nationalism and familial favoritism: Despite its mighty evolutionary basis, almost everyone recognizes moral strictures against familial favoritism. Almost everyone knows that “It would help my son” is not a good reason to commit murder, break someone’s arm, or steal. Indeed, almost everyone knows that “It would help my son” is not a good reason for even petty offenses – like judging a Tae Kwon Do tournament unfairly because your son’s a contestant.
Nationalism, in contrast, is widely seen as an acceptable excuse for horrific crimes against outgroups. Do you plan to murder hundreds of thousands of innocent foreign civilians ? Just say, “It will save American [German/Japanese/Russian/whatever] lives” – and other members of your tribe will nod their heads. Do you want to deprive millions of foreigners of the basic human rights  to sell their labor to willing buyers, rent apartments from willing landlords, and buy groceries from willing merchants? Just say, “It’s necessary to protect American jobs” in a self-righteous tone, then bask in the admiration of your fellow citizens.
It cannot be said too often or too loudly: one of the greatest curses that humankind has brought upon itself is the curse of nationalism.