Democratic Peace Theory should first and foremost be understood as a theory of politics and international relations, not a historical statement of fact. Its principles don't actually claim that war between two liberal democracies is impossible - rather, it exists to qualify and explain a clear statistical outlier: The fact that liberal democracies don't seem to go to war against each other (or at least not unless under highly unusual circumstances), and then seeks to explain why this may be so.

The theory variously goes that liberal democracies' populations wouldn't stand for a war, that their cultures are too intertwined, or that their economies are too intertwined. I think all of these in a vacuum can be disproved pretty neatly - we have examples aplenty of economically-linked countries going to war against each other, and we have plenty of examples of countries of a similar culture fighting (or indeed of the same culture, polity or nation, in fact - what we call a civil war), and we certainly have plenty of examples of wars between countries or polities whose populations might not have had nearly such great enmity as their rulers had for each other.

All the same, the fundamental thrust of the claim is true, despite the (in my view) occasional lack of depth as to the reason behind it. The simple fact is, broadly, liberal democracies don't seem to go to war with each other very often, even when they're neighbors, and even when they might have some legitimate grievances.

However! The reason I mentioned above that DPT is a political theory and not a statement of historical fact is because there are in fact several examples of wars between countries we would call liberal democracies, though crucially I think that these broadly serve to reinforce the political theory (the exception that proves the rule), despite denying DPT advocates the pure historical truth of the claim.

To look at the DPT claim that liberal democracies don't war on each other, we need to broadly ask what is a liberal democracy. I think it would be fair to say that we could anchor the time period looked at from 1750 on one end (a couple of decades before polities like the French Republic or the United States pop up, but a time in which already the United Kingdom and the Swiss Confederacy might reasonably be called democracies, and liberal for their time), all the way to 2000, as this is AskHistorians and we have a 20 year rule here and thus must cap our study.

Looking at that time period (of 250 years - wars abound within it) the DPT theorists are vindicated in that vanishingly few of them are between liberal democracies. But not quite none!

The Second World War made for some interesting circumstances, and provides us two decent examples that contravene DPT.

First, we have the British attack on the French fleet at Mers El-Kebir, as well as some other wider engagements in the Mediterranean in the wake of the Fall of France. The simplified version of the story is that, in the wake of the German capture of Paris and impending surrender of France, the Royal Navy attacked and destroyed the French fleet at Mers El Kebir to prevent it falling into German hands. As the Vichy government hadn't yet been formed by the time of the attack (48 hours away), the technical truth is that Britain attacked and destroyed an allied fleet belonging to the (last gasps of the) liberal French Republic. For obvious reasons, I think this does not present a cogent counter point to Democratic Peace Theory.

A slightly more difficult example is also from the Second World War - the Continuation War. In the Winter War between Finland and the Soviet Union, Nazi Germany offered assistance to the Finns; during the war that followed afterwards, known as the Continuation War, Finland and Germany fought against the Soviet Union in Finland. However, due to the circumstances of the wider Second World War, the Soviet Union was aided in its war on Finland (and Germany) by the United Kingdom. This 'strange bedfellows' situation meant that the United Kingdom and Finland, both liberal democracies, were at war with each other, allied as they were to the Soviet Union and Germany, respectively. Again, I don't think this presents a very cogent counter to DPT, though it is at least a more significant conflict than a single attack as was Mers El Kebir.

There is one example of two liberal democracies at war, however, that I think does raise some curious questions about how universally applicable DPT is - the War of 1812. While also set against the backdrop of major international turmoil (the Napoleonic Wars), the war was essentially instigated by one liberal democracy (the United States) attacking a constituent part of another liberal democracy (Dominion Canada, and thereby the United Kingdom), due to mutual grievances over economics, some maritime laws being broken, some reasonably imperious British naval behavior at sea, and historical grievances from the American War of Independence. The war was a significant manpower and military expenditure for both sides, and for the United States was potentially existentially threatening, as the British Army famously captured and destroyed the US capital of Washington DC. The war ended in a rather muted fashion with restoration of pre-war borders (with the exception of a few bits of land that went to Canada), and I think it is fair to attribute this relative restraint at least in part to shared cultures, shared views of liberalism, and shared economic interests. But the point still stands that two major, Western, liberal democracies fought a major war against each other with significant destruction and loss of life; it is the best counter example against DPT that I am aware of. DPT proponents, of which I fundamentally am one, will sometimes try to refute this example by claiming that one side or the other doesn't fulfill the requirements of being a liberal democracy, most often by claiming that Canada was not a liberal democracy as it was part of the British Empire. That's hogwash, in my opinion, as Canada had at the time as representative a legislature as the United States had, and it could certainly be argued that Canada was MORE liberal on many issues than the US, as the US at the time struggled mightily with issues on immigration, slavery and representation. Even beyond that, there's no defence in focusing on Canada's status, as it was the United Kingdom proper that sent troops to invade the continental US and capture cities like Washington, Baltimore and New York.

All the same, I think the War of 1812 is an excellent true example of the exception that proves the rule, because it is such an outlier. I also don't think it renders DPT obsolete at all; rather, I think it should inform us that DPT clearly has serious weight of evidence behind it, and is very valid, but is not the ultimate be-all end-all of geopolitics.

But there again, I'd be wary of any theory of international relations that claims complete universality; DPT is, despite not being quite universal, as robust and as well borne out by evidence as they come, in my opinion.

PART I

Ah, the Democratic Peace Theory. Not quite my least favorite political science theory - I guess that would be the Golden Arches Theory, although calling that a theory instead of "something Thomas Friedman thought up for an op-ed" is being generous. I did major in political science but its things like that that made me go into comparative studies, which was kind of a (second) history major by another name.

Anyway, just to restate the premise: the theory posits that democracies do not go to war with each other. The "soft" version of the theory is that democracies are less likely to go to war with each other, and the "hard" version is that democracies never go to war with each other. In either form it's a theory that has a relatively long pedigree (at least going back to Immanuel Kant), and ironically was part of the ideological reasoning for the US invading Iraq (if Iraq and by example its neighbors could be turned into stable democracies, there would be no more conflict in the region).

Part of why I dislike the theory personally is because it's basically unfalsifiable - we could provide a number of examples to disprove the theory, but there will always be counter-arguments as to why they don't "count".

A big issue comes from defining the terms. What is "democracy'? What's a "stable" democracy (this is the version I've heard although you can sub in OP's "mature liberal democracy" as well? What is "war" for that matter? Depending on how we define these terms, we can either find examples of wars, or rule out all examples.

First with "democracy". This is actually trickier than first appears. A lot of people (at least in Western Europe and North America), will probably read this as shorthand for "liberal multiparty democracies with regular elections and peaceful turnovers of power." So right there we're narrowing the definition of democracy to not include things like democracy under Marxist-Leninist regimes, which were democratic (in the sense of mass-participation in a political party and regular elections), just not in a way liberal democracies use the term.

But even for those liberal democracies with the definition I've provided, there are pitfalls - do we have to wait for the system to have its first peaceful transfer of power to be "democratic", or can the elections just include multiple parties and be generally considered "free and fair"? Does it matter if, say, it's a two party system which stacks election laws to favor them over other parties, even if those other parties theoretically can compete like the Big Two? Are we concerned about there being property qualifications or literacy qualifications to vote? Are women allowed to vote? Are minority groups under specific legal pressure to not vote or participate in politics? These questions matter because they give you radically different answers as to when you can draw the line: based on your answer, in the case of the United States, you would get a vast range of starting points for American democracy: 1789 (the start of the current constitutional order), 1800 (the first peaceful transfer of power between parties), 1832 (more or less when universal white male suffrage happened), 1920 (when women gained the right to vote), 1965 (when the Civil Rights Act was passed), or not yet (because of the two party duopoly, many continual challenges to minorities and lower income voters, disengagement with at least one half of the national electorate from voting, gerrymandering, the major parties representing the same brought capitalist interests, etc.). I have seen reasonable arguments made for all of those dates. You can make similar arguments about other countries as well: does the UK have a centuries-old parliamentary democracy? Or do we start counting from universal male suffrage and partial female suffrage in 1918? Or full female suffrage in 1928? Do we care about colonies? Etc.

Then there is the usual qualifier of a democracy being stable. Honestly, this one is the most weasel-y part of the definition because I'm not sure how you can objectively quantify "stability". That plus the fact that engaging in wars are by their nature instability, so this part of the definition leads to circular logic (stable regimes aren't unstable).