I kill ... I wound ... I will make mine arrows drunk with blood, and my sword shall devour flesh. Deuteronomy 32:39-42
Remember the Holy Civil War a few killings back? Well this is a lot like it.
It starts with the people from Jabeshgilead making a proposition to the Ammonites.
Then Nahash the Ammonite came up, and encamped against Jabeshgilead: and all the men of Jabesh said unto Nahash, Make a covenant with us, and we will serve thee. 1 Samuel 11:1
Does Jabeshgilead sound familiar to you? Well, if you read about God's 59th killing, it should. (In that story, the Israelites killed everyone in the city of Jabeshgilead except for the virgin women, whom they gave to the surviving Benjamites for wives.)
So everyone in Jabeshgilead was killed a few years before the events in 1 Samuel 11 supposedly took place. Yet here in verse 1 they are making a treaty with the Ammonites. Do dead people make treaties? I guess they do in the Bible.
Anyway, here's the deal that Nahash offered the (dead?) people from Jabeshgilead:
Nahash the Ammonite answered them, On this condition will I make a covenant with you, that I may thrust out all your right eyes, and lay it for a reproach upon all Israel. 11:2
Now you might think that this would be a 'no brainer' to the people of Jabeshgilead. But since they were already dead, maybe they didn't have any eyes to poke out. In any case, the people of Jabshgilead asked for a week to see if they could get an army together to fight the Ammonites. If they couldn't, they'd let the Ammonites poke out one of their eyes.
The elders of Jabesh said unto him, Give us seven days' respite, that we may send messengers unto all the coasts of Israel: and then, if there be no man to save us, we will come out to thee. 11:3
The Ammonites said, "Sure, go ahead and get an army together. We'll give you a week." So the leaders of Jabeshgilead sent messengers to Saul.
When Saul heard about it, "the Spirit of the Lord came upon him," and Saul did what any spirit-filled person would do: he killed some oxen, chopped their bodies up into 12 pieces, and sent the pieces to the 12 tribes of Israel.
The Spirit of God came upon Saul ... And he took a yoke of oxen, and hewed them in pieces, and sent them throughout all the coasts of Israel by the hands of messengers. 11:6-7a
And it worked, too. Messages like that always work in the Bible.
The fear of the LORD fell on the people, and they came out with one consent. 11:7b
Within a week the ox body parts were sent around to all the tribes of Israel and all the people of Israel responded "as one," forming an army of 330,000.
When he numbered them in Bezek, the children of Israel were three hundred thousand, and the men of Judah thirty thousand. 11:8
It's amazing what a twelve rotting pieces of meat can do!
Then Saul told the people of Jabeshgilead:
To morrow, by that time the sun be hot, ye shall have help. And the messengers came and shewed it to the men of Jabesh; and they were glad. 11:9
So the messengers went back and told the good news to leaders of Jabeshgilead, and they told Nahash that tomorrow they'd let them poke out their eyes.
The men of Jabesh said, To morrow we will come out unto you, and ye shall do with us all that seemeth good unto you. 11:10
The next day Saul and his army killed Ammonites until it got a bit too hot for killing. Then they stopped and took a little break. Before they were done, though, they had killed all the Ammonites.
Saul ... slew the Ammonites until the heat of the day: and it came to pass, that they which remained were scattered, so that two of them were not left together. 11:11
After the slaughter, some of the people wanted Samuel to kill all the Israelites that didn't want Saul to be made king.
But Saul said, There shall not a man be put to death this day: for to day the LORD hath wrought salvation in Israel. 11:13
So since it was God that did all the killing, he deserves all the credit.
The Bible doesn't say how many Ammonites were killed; I'll call it a standard massacre and say 1000.