Guardian,
" Exceptions to the democratic peace can be explained away and largely prove the rule. "

"To find an example of war between democracies we must go back to the Peloponnesian War. Athens's attack on Syracuse refutes the hypothesis, yet it is questionable whether the Athenians knew that Syracuse possessed a democratic polity or whether the rule of democratic peace applies to ancient warlike republics."
https://www.theguardian.com/notesandqueries/query/0,,-3134,00.html