Transitivity to this degree only counts for federations, I believe.
If Minas Leon came into a region with Talerium and Rieleston in it, they would fight only Talerium, because Rieleston would not be their ally, and thus would not be chosen as a defender.
I think that at this degree of separation, even if all five realms were in the same region, Rieleston and Talerium would never be chosen as defenders in the same battle.