[quote=NarcolepticSailor]
Well, the French were abiding by the Treaty of Constantinople I imagine in coming to the aid of the Austrians. Which gave them a proper reason to declare war, then the British are allowed to join as well through their existing alliance. Although the British themselves said that they were done with the Treaty, so I don't know.
[/quote]
Actually, we had a secret alliance for the specific purpose of staving off Otto if either of us ended up at war with Prussia.

Outcast mentioned something on the first page about secret alliances, so presumably, they're a legitimate cause for war.