Albert Barnes Commentary Judges 21:18

"Howbeit we may not give them wives of our daughters, for the children of Israel had sworn, saying, Cursed be he that giveth a wife to Benjamin." — Judges 21:18 (ASV)

For the children of Israel have sworn - See (Judges 21:1). Compare Saul’s rash oath (1 Samuel 14:24), and his breach of the oath made to the Gideonites (2 Samuel 21:2). For the guilt of a broken oath, see (Ezekiel 17:15–20); (Exodus 20:7).