Plutarch Demetrius 28

[1] But the fortunes and achievements of the man whose Life I am narrating, brings my narrative back, as it were, from the comic to the tragic stage. For all the other kings leagued themselves together against Antigonus and united their forces, and so Demetrius set forth from Greece, ¹ and finding his father eager beyond his years for the war, he was himself still more encouraged.

[2] And yet it would seem that if Antigonus had made some trifling concessions and had slackened his excessive passion for dominion, he might have always retained the supremacy for himself and have left it to his son. But he was naturally stern and haughty, and was harsh in what he said no less than in what he did, and therefore exasperated and incited against himself many young and powerful men; and their combination and partnership at this time he said he would scatter asunder with a single stone and a single shout, as if they were a flock of granivorous birds.

[3]
  He took the field with more than seventy thousand infantry, ten thousand horse, and seventy-five elephants; while his adversaries had sixty-four thousand infantry, five hundred more horse than he, four hundred elephants, and a hundred and twenty chariots. After he had drawn near them, the cast of his expectations rather than of his purposes underwent a change.

[4] For he was wont to be lofty and boastful as he engaged in his conflicts, making pompous speeches in a loud voice, and many times also by the utterance of a casual jest or joke when the enemy was close at hand he would show the firmness of his own spirit and his contempt for them; but now he was observed to be thoughtful and silent for the most part, and he presented his son to the army and pronounced him his successor.

[5] But what more than anything else astonished everybody was his conversing alone in his tent with his son, although it was not his custom to have secret conferences even with him; instead, he made his own plans, followed his own counsels, and then gave his orders openly. At all events, we are told that Demetrius, when he was still a stripling, asked his father when they were going to break camp; and that Antigonus replied in anger: Art thou in distress lest thou alone shouldst not hear the trumpet?