[dragonwriter]

> It's hard to prove a negative through evidence

No, it's not, in the usual scientific sense of proof rather than some absolutist one; you show the expected incidence of the inhibitory effect did not exist, and then show that the actual value (either by surveying the whole universe of concern or a random subset) is below that expected value to a degree that makes it improbable that it occurs by chance, while controling for other known sources of variability.

> but can you cite and exception?

Sure, if you use a definition of Democracy that isn't so narrow that the expected number of wars if democracies on both sides isn't far less than one, you will also find lots of actual wars between Democracies. Even if you ignore wars where the only pair of democracies are a separatist group and the unit they are separating from (e.g., the American Revolution), there are plenty (War of 1812, for instance, or more recently some parts of the Yugoslav Wars.)

Wikipedia has more discussion and examples:

https://en.m.wikipedia.org/wiki/List_of_wars_between_democra...

> have non-democracies ever achieved anything like the EU or NATO?

Yes, non-democracies have created both continent-spanning multi-national supergovernments like the EU and wide regional military alliances like NATO.

In fact, they did both long before democracies did.

[forapurpose]

It would be great if you could cite some examples. The American Revolution and War of 1812 clearly are not ones; the UK was not a democracy. The wars in the former Yugoslavia are a stretch; those were at best very nascent democracies.

> if you use a definition of Democracy that isn't so narrow ...

The word has a meaning; these aren't narrow definitions any more than saying 'Manchester United isn't a basketball team' depends on how narrow your definition is of 'basketball'.

[dragonwriter]

> The American Revolution and War of 1812 clearly are not ones; the UK was not a democracy.

At what point precisely did the Westminster system, one of the more common models of modern democracy, become a democracy?

Note that the later you peg and the more narrowly you define “democracy” to justify it, the more you make the case that there have been so few democracies that, given the actual historicsl frequency of interstate wars and the number of nations available at any time, the number of wars expected to have a democracy on either side is expected to be near zero even if democracy in both sides of a dyad does nothing to decrease the probability of war between the countries.

(Also note if you claim it's not a democracy because it had territories unrepresented in the national government despite the form of government being democratic, that has the same effect—and even rules out the modern US as a democracy, as well.)

[slededit]

The transitioned happened during the reign of Queen Victoria where the monarchy transitioned from a seat of power to a ceremonial role.