All commanders are hostile with one another in default setting. You can have personal diplomacy when either or both of you have no league.

If you apply to be allied with one commander, only with his/her approval can it come into effect.

If you apply to be neutral with a allied commander, it comes into effect immediately. 

If you apply to be neutral with a hostile commander, only with his/her approval can it come into effect.

If you apply to be hostile with one commander, it will come into effect in 30 minutes.

You can view and deal with personal diplomacy on Message interface. (Tap the envelope icon and choose Messages tab. )