Mmhmm.. the definition of a defensive pact is complicated, but that popular saying (It's a defensive, not an offensive, pact) is good.
Let's say there are 3 Civilizations: A, B, Z
A signs pact with B
Case I: If Z attacks A, then B must join in war against Z.
If Z attacks B, then A must join in war against Z.
Case III: If A attacks Z, then B does not have to join in war.
If B attacks Z, then A does not have to join in war.
Case V: If A attacks B, then the pact is cancelled, and Z does not need to join against either side.
If B attacks A, then the pact is cancelled, and Z does not need to join against either side.
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Now, Let's say there are 3 Civilizations: A, B, Z
A signs pact with B
B signs pact with Z
Z signs pact with A
If anyone declares war on anyone else {A,B,Z}, then the other two will automatically be at war with the aggressor.
So if A declared war on Z, then A's pact with Z is voided, and A's pact with B is voided. Now B's only association is with Z, and because Z is being attacked, B must join in war against A.
