OK - here's something I did tonight, started the game in the Old Gods as Basil the Macedonian, after dealing with the invasion of Syracuse (Roma Victrix) I decided to retake all of Sicily and at the same time strike at Krete and the Levant.
Here we are at the start of the war, note at this point I have made no conquests and therefore there is no Defensive Pact:
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I'll describe this as a "daring" attack and if one of the larger Muslim rulers decided to get involved this was going to get sticky. Fortunately everybody sat there Stupefied while I took one province in Italy and the Island of Krete:
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Here we are half way through, and this is what the Defensive Pact map looks like at this point:
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Here we are at the end, all those rulers defeated, it took a while but I was never really stretched. I unwisely murdered one of my vassals though, so I had to give over War Declaration to make sure I wasn't overthrown.
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Note my threat level at this point was 75.14%, which means the Christians will now go to war in support of the Muslims.
So, let's summarise:
1. The defensive Pacts did not form until after I actually annexed two realms, and they didn't prevent me from completely unlanding four characters, crippling two others and seriously denting a
seventh with what was, frankly, relative ease.
2. After my conquests against exclusively Muslims the largest and most cohesive pact is the Christian one.
3. Despite the fact that all those Christian realms are now safer (because there are no longer Muslim realms with Holy War CB within range) they will now go to war to prevent me from attacking more Muslims.
The problem with this isn't just that it's an arbitrary system or that you end up fighting the mechanic directly instead of the AI because everything is based on a fixed number which has nothing to do with character opinion, it isn't event he fact that the mechanic completely failed to even slow me down as I re-drew the map at the start of the game.
No, the problem is that the Defensive Pact mechanic has created a situation which it is simply impossible to rationalise, it's fatal to the sense of immersion (not a word I would normally use) because it's impossible to see this as anything other than a game mechanic. This might be forgiveable if it actually retarded player expansion, but it clearly doesn't.