Came across these great quotes of Twain's today. We all know this one, of course:

"Patriotism is the refuge of the scoudrel."

But here are some other choice ones on the subject:

The soul and substance of what customarily ranks as patriotism is moral cowardice--and always has been.
- Mark Twain's Notebook

We teach them to take their patriotism at second-hand; to shout with the largest crowd without examining into the right or wrong of the matter--exactly as boys under monarchies are taught and have always been taught. We teach them to regard as traitors, and hold in aversion and contempt, such as do not shout with the crowd, and so here in our democracy we are cheering a thing which of all things is most foreign to it and out of place--the delivery of our political conscience into somebody else's keeping. This is patriotism on the Russian plan.
- Mark Twain, a Biography

Man is the only Patriot. He sets himself apart in his own country, under his own flag, and sneers at the other nations, and keeps multitudinous uniformed assassins on hand at heavy expense to grab slices of other people's countries, and keep them from grabbing slices of his. And in the intervals between campaigns he washes the blood of his hands and works for "the universal brotherhood of man"- with his mouth.
- "The Lowest Animal"