Units from different armies, but which share the same alignment (i.e. either Order or Destruction – non-aligned units are always treated as being suspicious allies, as detailed later) are considered to be 'trusted' allies when they fight in an alliance. They are battling towards the same goal (or one so similar as makes no difference) and this common cause is likely reinforced by centuries-old oaths and the weight of a shared history.
Trusted Allies follow the rules for Bound by Blood except that they:
Cannot be joined by allied characters.
Cannot use an allied General's Inspiring Presence rule.
Cannot use an allied battle standard's Hold Your Ground! rule.