John Calvin on Jeremiah 5:20

The Prophet confirms what he had said, lest the Jews should think that they were only terrified by words, and not dread the consequences. Hence he says, Declare this The Prophet, no doubt, alludes to a custom which prevailed; for wars were usually proclaimed by heralds. Enemies did not immediately march forth, but they proclaimed war that the cause might appear just. Hence God here declares, that he had spoken in earnest by the mouth of Jeremiah, as though war had been in the usual manner proclaimed, and armed enemies were already nigh at hand.