When a vast army came against him, what did King Jehoshaphat proclaim throughout Judah?

Answer: A fast

Alarmed by the armies of Moab and Ammon, Jehoshaphat set his face to seek the LORD and proclaimed a fast throughout Judah.

“Then some men came and told Jehoshaphat, “A vast army is coming against you from Edom, from beyond the Sea; they are already in Hazazon-tamar” (that is, En-gedi). Jehoshaphat was alarmed and set his face to seek the LORD. And he proclaimed a fast throughout Judah.”— 2 Chronicles 20:2-3 (BSB)