Titus Livius (Livy), The History of Rome, Book 40
Evan T. Sage, Ph.D. and Alfred C. Schlesinger, Ph.D., Ed.

21. Philip, in order partly to prevent his troops from growing slack through idleness, partly to avert [p. 67]suspicion that he was thinking at all about a war¹ with Rome, summoned his army to Stobi in Paeonia and began to lead it into Maedica. [2] The desire had seized him of climbing to the top of the Haemus mountains, because he had accepted the common opinion that from there could be seen all at once the Pontic and Adriatic seas, the Hister river and the Alps. To have these spread out before his eyes would have, he thought, no small weight in determining his strategy in a war with Rome. [3] Having questioned men who knew the country about the ascent of the Haemus, since it was generally agreed by all that there was no way for an army, but that with a few light troops it could [4??] be climbed by a very difficult road, wishing to console his younger son by an intimate talk (he had decided not to take him with him), he first asked him whether, since the difficulty of the journey was so great, he should persist in the plan or abandon it. [5] If after all he should decide to go, he could not forget, in such a situation, Antigonus,² who, when tossed about by a dreadful storm, at a time when he had all his family in the same ship with him, was said to have given his children the warning, which they should both remember for themselves and pass on to their children, that no man should venture to endanger himself and his whole house at once in a doubtful situation. [6] Therefore, he said, remembering that precept, he would not expose both his sons at once to the chance of disaster which confronted them; and since he was taking his older son with him, he intended to send the younger back to Macedonia to furnish some support to his hopes and for the future to guard the kingdom. [7] Demetrius did not fail to [p. 69]perceive that he was being banished, in order that³ he might not be present at the council when the decision would be reached, in sight of the theatre of operations, what routes would lead most directly to the Adriatic sea and to Italy, and what would be the general strategy of the war. [8] But he had not merely to obey his father but to agree with him, lest he should beget suspicion that he was obeying unwillingly. [9] Nevertheless, in order that he might have a safe journey into Macedonia, Didas, one of the king's generals,⁴ who was in charge of Paeonia, was ordered to escort him with a moderately large guard. [10] Didas too, like most of the friends of the king, from the time when it began to be clear to everyone to whom, since the king's mind inclined that way, the inheritance of the kingdom would fall, Perseus had associated with the conspiracy for his brother's ruin. [11] For the present, he instructed Didas to make for himself, by every form of attention, a place among the intimates of Demetrius, that he might be able to get from him all his secrets and to spy upon his inmost thoughts. Thus Demetrius departed, accompanied by guards whose presence was more dangerous than if he had been travelling alone.