Albert Barnes Commentary 2 Chronicles 20:24

"And when Judah came to the watch-tower of the wilderness, they looked upon the multitude; and, behold, they were dead bodies fallen to the earth, and there were none that escaped." — 2 Chronicles 20:24 (ASV)

The march of Judah from Jerusalem would take five or six hours. By the time they reached the watchtowers in the wilderness of Jeruel, it was all over.