John Gill Commentary 1 Samuel 14:20

"And Saul and all the people that were with him were gathered together, and came to the battle: and, behold, every man`s sword was against his fellow, [and there was] a very great discomfiture." — 1 Samuel 14:20 (ASV)

And Saul, and all the people that were with him, assembled
themselves
The six hundred men that were with him, unless we can suppose the 1000 that had been with Jonathan in Gibeah were here still, see (1 Samuel 13:2)

and they came to the battle ;
to the field of battle, the place where the army of the Philistines had lain encamped:

and, behold, every man's sword was against his fellow ;
taking one another for Hebrews, or treacherous and disaffected persons; so that, though the Israelites had neither swords nor spears, they needed none, for the Philistines destroyed one another with their own swords; and there was a

very great discomfiture ;
noise, tumult, confusion, slaughter, and destruction.