The whole of Israel would have already known about the harsh consequences of making foolish vows, but those things are easy to forget in the excitement of battle. Just as Jephthah lost his daughter, Saul would have lost his son, the hero of the day, but for the intervention of the people.
As before, the vow didn’t make God do anything he wouldn’t have anyway. By the time Saul’s troops entered the battle, the Philistine troops were already in trouble: an outpost had been defeated by two men (Jonathan and his armour bearer), the entire Philistine army had inexplicably panicked and turned on itself, and then an earthquake had struck. Saul’s role was perhaps more of a mopping-up operation that took all day and went into the evening.
And the vow? It didn’t win a battle; it just created an exhausted army:
1 Samuel 14:24 Now the men of Israel were pressed to exhaustion that day, because Saul had placed them under an oath, saying, ‘Let a curse fall on anyone who eats before evening—before I have full revenge on my enemies.’
Jonathan, unaware of the oath (probably because he was fighting Philistines while Saul was getting his troops in order), ate some honey. When told about the vow, his view was that his father had just ‘made trouble for us all’.
Forced to admit it at the end of the day, Jonathan had to ask whether that was really something he should die for. The response:
1 Samuel 14: 44-45 ‘Yes, Jonathan’, Saul said, ‘you must die! May God strike me and even kill me if you do not die for this.’ But the people broke in and said to Saul, ‘Jonathan has won this great victory for Israel. Should he die? Far from it! As surely as the Lord lives, not one hair on his head will be touched, for God helped him do a great deed today.’ So the people rescued Jonathan, and he was not put to death.