Isaiah 16:13-14. “This is the word that the Lord hath spoken concerning Moab since that time. But now the Lord hath spoken, saying, Within three years, as the years of an hireling, and the glory of Moab shall be contemned, with all that great multitude: and the remnant shall be very small and feeble.”
In these two final verses of Isaiah’s lament over Moab, God reveals that it will be three years from the time Isaiah made these warnings before Moab falls to Assyria. “As an hireling” is confirmation of the three year period. It was the custom for a hireling to have a three-year contract with his employer. At the end of three years, the hireling could choose to continue, or to leave.
The number of survivors would be very small, merely a remnant; that remnant would be weak, unable to defend themselves, feeble and without hope.
And that is exactly what happened. You can read the story in 2 Kings 17, in which Assyria takes the 10 northern tribes of Israel, along with Moab, into captivity.