As said, a gentleman of the first water. His like we'll not see again.
'Marching as to War' he wrote because when talking to a class in a secondary school in Melbourne one Anzac Day, a boy of Greek parentage asked him 'how come you Australians went to the defence of another country in Europe?'
Don did his best to get through to the lad about the empire and the crown and all the Anglo connections. His explanation made no impact on the boy or the class at all. So he went home and sat down and wrote a masterly account of the motivations leading to enlistment.