So I'm pulling a necro here to try to find the answer to this - I've looked through the forums and cannot find a response.
My situation - attacked Crimea as Russia. 2 weeks into war, Crimea makes alliance with Ottomans, who then get called in (didn't think that could happen, but here we are). Ottomans become war leader due to "massive blob effect". I manage to hold off 120k Ottomans while I siege down Crimea, get them to peace out. Hold off Ottomans another 3 months, get them to peace.
Fast forward 5 years to the day before the date of the peace with Crimea. Ottoman guarantee independence of Crimea.
Next day - I now have no truce with Crimea, and 3 months left on truce with Ottomans.
If I attack Crimea, does Ottomans get called in (similar to defender), or do they have to actively join? Does this count as breaking truce, and will the AI do it? Can they wait 3 months and then get involved?
I would normally just test this out myself, but its an ironman game and I would rather not screw it up with a massive war with the Ottomans over 2 provinces if I don't have to...
Thanks