Jephthah attempts to reason with the king of Ammon fail. The Spirit of the Lord comes upon him and he leads the successful charge against the Ammonites, but not before making a foolish pledge to God.
Scholars debate how Jephthah fulfilled his vow - was his daughter sacrificed as a burnt offering or was she set apart for service to God (not killed)? Either way, her future was altered by a promise he made and fulfilled. The message for us is clear: Don't make foolish promises based on future events.
"Do not be quick with your mouth, do not be hasty in your heart to utter anything before God." (Ecclesiastes 5:2)
God desires your obedience now, not your promise of future obedience. When facing life's challenges, don't make deals with God. Trust Him, the Judge, to look out for your best interests. Demonstrate your faith. If it's meant to be, it will be.
I'd rather trust and have things not work out according to my desires than to be explaining to God "I didn't mean it" when I promised something rash in the heat of some personal battle. How about you?
(8/4: I just read Proverbs 20:25 and couldn't help but add it to this post, considering how remarkably apropos it is: "Don’t trap yourself by making a rash promise to God and only later counting the cost.")