The forces of the king were reduced

I wonder how the forces of the king were reduced. Had they been reduced through fighting with the Lamanites? Had they been reduced because Noah was convinced (through hubris) that they could do with fewer guards?  Had they been reduced by some of them being converted by the preaching of Alma (which could also explain how some was appraised of the coming of the army)?

It is interesting to think as Gideon went to slay Noah with the sword that he couldn’t have succeeded because it had been prophesied that Noah would suffer death by fire.