Hey people, quick question here.
In my current game, I'm seeking to invade Spain. I managed to get France to ally with me, however they're also still allied to Spain, thus won't answer my call to war and would probably join the war against me should I declare directly on Spain.
However, Spain is also allied to Portugal and Tuscany, both of which France WILL accept a call to war for.
My question is as follows: If i declare war on Portugal (Tuscany will probably cause a warleader shift so no-go), and Spain immediately comes in to defend them, will France still answer my call? I'm asking because I get the feeling that sometimes, other allies joining a war can cause a CTA to a certain ally to fail, even though the diplomacy screen shows a check mark, and them not joining would change things up quite heavily (not disastrous, but would certainly make an invasion to the Iberian peninsula itself impossible).
The first answer is yes. The pessimistic answer is no.
9 out of 10 times, I've used the same strategy you've described to get two allies in a war together and it works. The 1 out of 10 however, something bugs out. Despite showing they would join, they decide against it anyway, declining the call to arms. I would still be at war with the war goal, but I wouldn't get any assistance from my allies while they get theirs.
So I want to say this strategy is sound and the best way to deal with allies allying with your enemies, but it's not foolproof.
If you do want to make it foolproof though, make sure you declare a war on someone you're gaurenteed to win against with little resources, and call in France. While France is an ally of yours against a small nation, declare war on Spain. France can also be called to war to this, because they cannot accept the defensive call to arms from Spain because they are allied with you in a war.