Set out to attack Ai: This first sentence provides a summary. Details follow in the next paragraphs.
The Hebrew words translated 30,000 might mean 30 units (see study note on Exodus 12:37) or 30 chieftains (leaders of groups or tribes). As the battle continued, this group waited in hiding for a surprise attack (an ambush). They remained hidden until Joshua called them into action (Joshua 8:18–19).