[quote=So Boerd]
Assuming a perfectly rational France, you wouldn't want either Austria nor Prussia strengthened. A balance of power Austria threatened by DOWing me.
[/quote]
I think we all know the war was coming eventually. France siding with Austria here is the logical choice, especially if Austria has no German ambitions of its own.

>Defeat Prussia.
>Put a buffer state in the Rhineland.
>Put a buffer state in Belgium.

Logically, France would want to surround itself with states that can't do anything to it. A unified German state would be the last thing France wants, and as long as Austria is interested in maintaining the status quo, i.e. no Germany, it's a logical alliance.