John Gill Commentary 1 Samuel 17:21

"And Israel and the Philistines put the battle in array, army against army." — 1 Samuel 17:21 (ASV)

For Israel and all the Philistines had put the battle in
array
Both sides prepared for it, and drew up in line of battle:

army against army ;
rank against rank, battalion against battalion, the right wing of the one against the left of the other