I'd like to see two democratic countries at war, sometime after 1900. It should be a long war, lasting at least three years. I think a potential candidate might be Japan, wherein the Taisho democracy lasts and coexists with expansionist tendencies, and it goes to war with either the United States or a democratic European country.
The definition of democracy will be contentious, I'm sure. I'm looking at a liberal democratic country, with a rather sizeable electorate. It's got to be a shooting war, not a Cod War. There have been aggressive actions between democratic countries before in the twentieth century, like the United States versus Guatemala in 1954, but nothing that's gone on too long.