As to the improbability of a general repentance in Nineveh itself, this must now be tempered by the recognition that our historical evidence preserves the fact that things were not going well at all for the Ninevites at approximately the time Jonah served as a prophet (the first half of the eighth century). Military and diplomatic losses internationally were coupled with famine and popular uprisings domestically during the time of Aššur-dān III (773–756 b.c.), for example. In addition, both an earthquake and an eclipse, dreaded major omens to the Assyrians, were experienced concurrent to these other problems. A weak, shaky monarchy reeling from domestic and international turmoil could well have welcomed the chance to solidify its acceptance by a suspicious populace, already set on edge by the prevailing problems, via the sort of royal proclamation preserved (in part?) in 3:7–9.
Word Bible Commentary, Douglas Stuart