Polybios Histories 9.35
"Not being able to say anything in defence of
any of these acts, you talk pompously about your having resisted the invasion of Delphi by the barbarians, and allege that for this Greece ought to be grateful to you. But if for this one service some gratitude is owing to the Aetolians; what high honour do the Macedonians deserve, who throughout nearly their whole lives are ceaselessly engaged in a struggle with the barbarians for the safety of the Greeks?
For that Greece would have been continually involved in great dangers, if we had not had the Macedonians and the ambition of their kings as a barrier, who is ignorant? And there is a very striking proof of this.
Defeat and death of Ptolemy Ceraunus in the battle with the Gauls, B.C. 280. See Pausan. 10.19.7.
For no sooner had the Gauls conceived a contempt for the Macedonians, by their victory over Ptolemy Ceraunus, than, thinking the rest of no account, Brennus promptly marched into the middle of Greece. And this would often have happened if the Macedonians had not been on our frontiers.