Patriotism vs. Nationalism
Nowhere have I read the distinction made clearer than in this post by Daniel Larison: The Intractable, Invincible Ignorance of Jonah Goldberg.

First, an absolutely incorrect assessment is made by the subject of the post's title:

For the record, John Lukacs has many great observations about the differences between patriotism and nationalism. The difference, to me and I believe to him, is that nationalism is rooted in the mystic concept of a nation—most famously in blood and soil—while patriotism is rooted in adherence to a creed or doctrine. A patriot in the Weimar Republic was considered a traitor by most nationalists, for example.

Here is the correction, from Scott P. Richert's THE ROCKFORD FILES:

In other words, patriotism is rooted in a particular place, and the people who live there, not in “adherence to a creed or doctrine.” By “a particular way of life,” Orwell (and Lukacs) mean just that: not abstract credal principles but the real life of real people in a real place—their language, their food, their religion, their manners, etc.

Nationalism, on the other hand, is, for Lukacs (and Orwell), an ideological phenomenon. It subsumes man in the nation; it divorces the nation from “a particular place and a particular way of life”; it defines the nation at least in part in terms of its opposition to the other.

Finally, Mr. Larison paraphrases Erik Ritter von Kuehnelt-Leddihn (1909–1999):

[T]he words themselves indicate the difference: the patriot loves his fatherland (Lat., patria, Gr., patris), something distinct and different from himself, while the nationalist identifies with and loves those like himself, which K-L maintained was more like self-love than real love.

Reader Adam Goldsmith once told me that Kuehnelt-Leddihn indentified Nationalism as Leftist and Patriotism as Rightist.