Helping an ally

[DVX BELLORVM]

If your 're-enforcing' your ally (in the message pop-up where they ask for your help), then you count as the aggressor, unless you reject it, then no war is declared.

That's right, the only thing is, there was no pop-up message, only the battle deployment screen...

No Then you are off the hook.

I tried reloading from the point before the battle, and even if I withdrew, the war starts