Remember in Judges when they killed everybody except the virgin women in Jabeshgilead to give wives to the remaining Benjaminites? Well, suddenly the Ammonites attack that exact same village and the men try to negotiate a treaty. Jerry Falwell, despite acknowledging in his annotations that this is indeed the same village, does not provide an explanation. So much for literalism. I'm willing to write it off as contradicting oral histories.
Nahash, the Ammonite king, is only willing to make a treaty on the condition that he can gouge out the right eyes of all the men. It's disgusting and yet it's still more humane than what the other Israelites did to them just one book ago.
The village elders ask for a week to see if they can't rustle up some support, and then they'll agree to the eye-gouging. Their messengers come to Saul's village, but rather like George W. Bush, Saul is tilling his fields. His people hear the news from Jabesh and weep in sympathy. Saul finds them like that and the spirit of the lord comes upon him. It causes him to hack a pair of oxen to death and send them around Israel as a message to the other tribes that the same fate will befall their cattle if they don't come along and help. Well, better cows than a concubine, I suppose, but is paper really in such short supply that they have to keep sending these grisly meat messages?
He manages to gather an army of 300 000 and sends the messengers back to Jabesh with the happy news. They arrive in the morning and massacre the Ammonites until it gets too hot. But some people are so bloodthirsty that no amount of killing will do, and they call for the people who were skeptical of Saul before, so they can kill them too. Saul says no, this killing was salvation for the traitors, and let's go to Gilgal instead. I like to imagine that since it was a hot day, Gilgal had something like a water park so they could cool down, but no, they crown Saul and make sacrifices. I suppose a barbecue is nice, too.