The Austro-Hungarian empire was also ruled by a hereditary monarch who was in charge of war and foreign policy.
I admit it's all a bit hand-wavy, but wasn't there a huge difference in nature between the likes of UK and France vs Germany, Austia-Hungary, and the Ottomans?
I wouldn't want this to become a tit for tat about who was the real democracy in the war: I'm perfectly happy to say that every participant fell far short of democracy. But the democratic peace folks are making a seemingly predictive, scientific claim using a really fuzzy and emotionally laden term like democracy that's easy to bend to mean whatever you like.
If there's something that's meant beyond my "US/UK and their allies don't fight each other" idea, I'm all for hearing it out. What are some of the countries considered democracies by the democratic peace proponents that aren't in that alliance and seem not to have been in a war with that alliance due to the fact that they're democratic and not autocratic?
I do concede that there it isn't inherently an error to say one country is more democratic than another, and that the Ottomans and Austria Hungary clearly end up on the losing side of that comparison.
You wouldn't imagine a civil war amongst a homogenous culture. I wonder if the Parliamentarians/Royalists or the Unionists/Confederates were more culturally different than the Anglosphere cultures are today.
Germany had tiered suffrage until the 1918 revolution.
"This distribution meant that a first-class vote had 17.5 times the value of a third-class vote." and "... one result of which was that the industrialist Alfred Krupp was the only person able to vote for the electors in the first class in Essen."
Consider WW1, in which most of the major belligerants, except for Tsarist Russia, have a plausible case for being called democratic. You might try to exclude one country or another, to save the Democratic Peace. But then you've got to be very careful about that: you can very easily add enough conditions until virtually no states until the 1970s were democratic, which gives you a small sample that coincides with an era when all states, democratic or not, have been relatively peaceful.
It also doesn't have a strong causal explanation for why it'd be the case. Democracies certainly aren't less violent in general: they don't have any issues fighting with autocracies. There's even evidence that weak or developing democracies, like Meiji Japan in the 1890s or the United States in the 2000s, actually are more violent than comparable states.
Most Democratic Peace theorists don't really offer anything more powerful than "the US/UK axis and its allies don't go to war with each other." Which is true, but uninteresting.