What happens when you go to war with a nation that's allied with your allies?

[Amaz]

I want to know the consequences of that happening...

[klesh]

I have done this and it seems that a mutual ally will stay out of the fight. That being said, make sure the ally likes you better than your enemy or the case may be different.

[Paulverisor64]

In the campaign I just completed my Ally dropped me and went to war against me. All the major factions even this ally was Hostile in disposition against me however.

[Amaz]

well, Brittain opinion towards me is indifferent. I don't know it's opinion towards my enemies though.

[Mahatma Gandhi]

But, you can find out simply by diplomaticing their name, and scrolling over the other country. It'll give you their relationship rating w/ea other. Either way, Klesh is right, they generally either pick a side or stay completely out of it. And, personally, I think that this is where religion plays a major factor. If you're opposing religion wise to the other ally, but the other 2 match, expect them to go the other way.

[Deathofjams]

Also, the balance of power plays a role. I (as the Ottomans) was trying to invade Poland without Russia (my, and their, ally) getting involved, and the first time I tried I ended up with both attacking me. However, once I got an alliance with Prussia and tried again, the Russians ended up joining me and all three of us pounced on Poland and ripped her apart. Ended up with some awesome three-one-one battles, the only time I've properly fought alongside allies in all three of my completed campaigns.

So yeah, make sure your relationship is better and that you've got a few good allies backing you up already.