Mutual Defense and Optional Aggression Pacts (often referred to as MDoAP's) are a type of interalliance treaty wherein the signatory alliances agree to defend each other from attack by other alliances. Optionally, they may support each other in wars of aggression, but this is not required. There is also a variant of the MDoAP and you may see it sometimes as MnDoAP. MnDoAP is the acronym for Mutual Defense and Optional Aggression Pacts that have a non-chaining clause in them, (here's an example of a non-chaining clause). The non-chaining clause is put into some MDoAP or MADP's so that if one signatory assists another alliance via a separate treaty, the treaty with the non-chaining clause becomes optional. These are used to limit to expansion of conflicts if people don't want to be involved for one reason or another.