NeGoc 5 Rules

[Loser]

An elder race is not allowed to break treaty with any minor before turn 120.

However, if a minor breaks treaty with an elder, the elder still cannot attack the minor, but only allow combat to happen where it occurs by accident or when initiated by the minor.

Unless the minor has property in the homesystem of the elder, in which case the elder may attack it with impunity.

[Tanus]

As far as I understand it, if a minor comes into an ancient's homesystem, and will not remove the ships, the ancient is allowed to break the treaty for the purpose of destroying the offending ships, but then must re-establish the NA treaty (or at least try to)