[quote=The Nexerus]
Yes. Their alliance is a perfectly genuine reason for going to war together, just like it was against the Union.
[/quote]

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.