I kill ... I wound ... I will make mine arrows drunk with blood, and my sword shall devour flesh. Deuteronomy 32:39-42
After Jephthah finished killing and burning his daughter for God (53) to pay God back for helping him slaughter twenty Ammonite cities (52), he ran into some Ephraimites who were angry about being left out of the Ammonite slaughter.
The men of Ephraim gathered themselves together, and went northward, and said unto Jephthah, Wherefore passedst thou over to fight against the children of Ammon, and didst not call us to go with thee? Judges 12:1a
They were so pissed off about the whole thing that they threatened to burn down Jephthah's house.
We will burn thine house upon thee with fire. 12:1b
Jephthah claimed that he invited them to join him in the God-assisted slaughter, but they didn't come.
Jephthah said unto them, I and my people were at great strife with the children of Ammon; and when I called you, ye delivered me not ... and the LORD delivered them into my hand. 12:2-3
Clearly, there was only one thing for Jephthah to do: call for a holy civil war.
Then Jephthah gathered together all the men of Gilead, and fought with Ephraim. 12:4a
So that's what he did, and the men from Gilead defeated the Ephraimites.
And the men of Gilead smote Ephraim. 12:4b
After the battle, Jephthah posted guards at the Jordan River where the fleeing Ephraimites would have to cross.
The Gileadites took the passages of Jordan before the Ephraimites. 12:5a
When an escaping Ephraimite would come to the crossing, the Gileadites would ask him if he was an Ephraimite. If he said, "No", they'd ask him to say "Shibboleth". (Ephraimites couldn't pronounce it correctly. It was like asking former President Trump to pronounce "anonymous".)
When those Ephraimites which were escaped said, Let me go over; that the men of Gilead said unto him, Art thou an Ephraimite? If he said, Nay; Then said they unto him, Say now Shibboleth: and he said Sibboleth: for he could not frame to pronounce it right. 12:5b-6a
Then when the Ephraimite mispronounced Shibboleth by saying Sibboleth, they'd kill him.
Then they took him, and slew him at the passages of Jordan. 12:6b
And the Shibboleth test worked like a charm. 42,000 Ephraimites failed the test and were killed trying to cross the Jordan.
there fell at that time of the Ephraimites forty and two thousand. 12:6c
And they deserved it, too, for not participating in the slaughter of the Ammonites, threatening to burn down Jephthah's house, failing to pronounce "Shibboleth" correctly, or whatever.
(Note: Jephthah is one of the heroes of God in Hebrews 11, so we can be pretty sure that God approved of the whole "Shibboleth" test / holy civil war massacre.)