Jamieson, Fausset and Brown on Jeremiah 42:16

16. sword, which ye feared, shall overtake you--The very evils we think to escape by sin, we bring on ourselves thereby. What our hearts are most set on often proves fatal to us. Those who think to escape troubles by changing their place will find them wherever they go (Eze 11:8). The "sword" here is that of Nebuchadnezzar, who fulfilled the prediction in his expedition to Africa (according to Megasthenes, a heathen writer), 300 B.C.