Do Allied nations join war together?

[Haig]

It says in Civilopedia that every type of Alliance gives it's members automatically Open Borders and Defensive Pact.
So if I'm allied with Spain and they are attacked, I join them in war.

But what if my ally Spain goes to war against the Zulu, do I join the war also automatically?

 

[Eva-kun]

No. You only go to war together if another civ declares war on either of you, if said civ not a friend with one of the civ in the alliance. Defensive pact in alliances was a good addition. If you want to go to war against one civ. Use the joint war feature.

 

[clapyourhands]

Eva-kun explained it well-enough. It's just as the name says, a Defensive Pact--it takes effect defensively, but not offensively. Historically this is pretty much how most alliances worked--peacetime allies might be called called into war if an alliance member is attacked, but negotiating an offensive attack was another matter entirely. From a game standpoint, it would suck to have an ally declare war on, say, one of your neighbors while being far away themselves to drag you into a borderfront war that doesn't affect them at all.