It would be hard to see Capaldi working with and respecting Captain Jack after the way he treated "PE".

But Jack's an old friend and would understand what 11 and 12 have been through on Trenzalore and since. Danny had his own problems with the Doctor and they played off each other, more or less fighting over Clara.
Thank goodness she's finally gone and can't cause any more trouble

Hence my suggestion of a desperate 12, as yes the military option would have been VERY out of character, especially in S8.