The Hundred-year war didn't break up British or French society. And that's exponentially longer than our longest one.
True. So you're saying "Long wars are not antithetical to Democracies."
I think technology has a lot more to do with the previous ability to wage longer war with less political capital than does politics itself. In the past you didn't have My Lai imagery (a transformative event in warfare and politics) spread across the society as easily, they were less connected than people are today. Additionally, economics were very different and economic contagion today is another thing that makes for much less tolerance for war (you could also say that long wars are antithetical to capitalism in a more global market).
Long wars are
antithetical to societies, not just democracies, of course. But in a democracy war weariness can
grow more easily than in political systems where the press is not free and where the leaders do not have to answer to public opinion.
These fundamental differences make a huge difference in the political capital it takes to wage war. If public outrage is only going to meet your iron fist anyway you can afford to wage an unpopular war more easily than the guy who needs to be reelected in four years (and war is an easy platform to campaign against) and who can't squelch the press.