Jamieson, Fausset and Brown on Jeremiah 44:29

29. this ... sign unto you--The calamity of Pharaoh-hophra (see on Jr 44:30) shall be a sign to you that as he shall fall before his enemy, so you shall subsequently fall before Nebuchadnezzar (Mt 24:8) [Grotius]. Calvin makes the "sign" to be simultaneous with the event signified, not antecedent to it, as in Ex 3:12. The Jews believed Egypt impregnable, so shut in was it by natural barriers. The Jews being "punished in this place" will be a sign that their view is false, and God's threat true. He calls it "a sign unto you," because God's prediction is equivalent to the event, so that they may even now take it as a sign. When fulfilled it would cease to be a sign to them: for they would be dead.