Then I said, “Ah, Lord GOD, how completely You have deceived this people and Jerusalem by saying, ‘You will have peace,’ while a sword is at our throats.”
Some so understand this passage as though the Prophet brought forward what was said by the people; for all the most wicked, when oppressed by God’s hand, usually cast the blame on him, and in their complaints contend and dispute with him. Hence they think that the Prophet here, not in his own person, but in that of the whole people, speaks thus: “O Lord, what can this be?
God's usual method is to warn before he wounds. In these verses, accordingly, God gives notice to the Jews of the general desolation that would shortly be brought upon them by a foreign invasion. This must be declared and published in all the cities of Judah and streets of Jerusalem, that all might hear and fear, and by this loud alarm be either brought to repentance or left inexcusable.
Commenting on Jeremiah 4:5-18
Then said I, ah, Lord God!.... Expressing great sorrow and concern: this "ah" is by way of lamentation. The Targum interprets it as a petition, "and I said, receive my prayer, O Lord God:'' surely thou hast greatly deceived this people and Jerusalem: what the false prophets did, that God is said to do, because he suffered them to deceive the people; see Kg1 22:20.