Here's another brief excerpt from John Lukacs' fine book on George Kennan. Lukacs notes that Kennan "was a patriot and not a nationalist: because patriotism is defensive, while nationalism is aggressive; because patriotism is traditionalist, while nationalism is populist; because patriotism is love of one's land and history, while nationalism is a viscous mass that binds formless masses together. A patriot will be concerned with his nation's faults (Chesterton: "my country right or wrong is saying: my mother drunk or sober")."
Somebody once said that patriotism is love of country while nationalism is dislike of other countries. There's truth there, but I think Lukacs' formulation is a little more elegant.