The member has posed a very challenging question, which is fair enough. If nationalism can be held in check only by authoritarianism, that tells you something quite important. It tells you that there are genuine, indigenous, popular sentiments that are being suppressed forcibly, and I don't think anybody's in favour of that. The old Soviet Union had its nationalities policy, a great preoccupation of both Lenin and Stalin, and there was every effort to repress national sentiments, which ultimately failed. It turned out they were not suppressed so much as cryogenically frozen, and as soon as the post-1989 thaw occurred, there were the same old divisions.
Identification with one's nation is an important source of identity for many people. It can be a source of unity among an otherwise disparate people. There is no contradiction between nationalism and liberal democracy. As a matter of fact, in the 19th century, the two were twinned. If you remember thinkers and actors like Mazzini, for example, the whole idea was that liberal democracy and national liberation would go hand in hand.
Can nationalism lead to excesses? Of course it can, and it has, but the suppression of nationalism can also lead to excesses, and it has. There is no simple formula for managing this force, but it is there. It can't be denied, and therefore, a far-sighted defence of liberal democracy of the liberal international order will do its best to make peace with nationalism and not pretend that it doesn't exist or that it's going away, because it isn't.