And Jephthah made a vow to the Lord: “If you give the Ammonites into my hands, whatever comes out of the door of my house to meet me when I return in triumph from the Ammonites will be the Lord’s, and I will sacrifice it as a burnt offering.”
Judges 11:30-31 (NIV)

Here we encounter a most difficult situation. A man vows to the Lord and ends up sacrificing his own daughter. How do you read this and not feel at least a little tense, with the need to grasp at anything to help you work this all out? Since we don't want to just skip through Scripture, we have to face it head on. With the Holy Spirit leading us, we can come to a place of understanding.

The question we are looking to answer is this: why does God let a man sacrifice his own daughter? Would God even accept this sacrifice? Why does God not intervene? These questions are almost inevitable when reading such passages and can easily extend into all matters of life. Why did God let (fill in tragedy) happen, and why did He not intervene? How you answer these questions depends largely on your thoughts and understanding of God and how He works. In the grander scheme of all things that exist or happen, God has let them be. Included in what He has let be is human will. Human will always carries with it a consequence.

Do not be deceived: God cannot be mocked. A man reaps what he sows. Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life.
Galatians 6:7-8 (NIV)

In keeping with this understanding, some things that happen are not happening because of God's direct influence, but rather in order to God's set natural law. The natural law also functions within the confines of a fallen world. Now back to our narrative - Jephthah has lived away from his father's house after being banished, because his mother was a harlot. He then ends up banding together with a group of worthless rebels and becomes their leader. With the challenges that he has faced in life, it is possible that his belief in God's ability and or desire to protect Israel has waned and thus he makes a vow. Now, vows were common practice at the time; however, they did carry an element of faithlessness, which is why Christ comes and says this:

“You have also heard that our ancestors were told, ‘You must not break your vows; you must carry out the vows you make to the Lord.’ But I say, do not make any vows! Do not say, ‘By heaven!’ because heaven is God’s throne. And do not say, ‘By the earth!’ because the earth is his footstool. And do not say, ‘By Jerusalem!’ for Jerusalem is the city of the great King. Do not even say, ‘By my head!’ for you can’t turn one hair white or black. Just say a simple, ‘Yes, I will,’ or ‘No, I won’t.’ Anything beyond this is from the evil one.
Matthew 5:33-37 (NLT)

We recognize here that Jephthah's vow has not come by inspiration from God. He knew that the LORD provided victory to Israel - the Spirit of the Lord was already upon him, negating the need to make a vow by which to curry God’s favor.

What follows after the victory is sad, but not of God's making. To say God should have stopped the daughter from coming out or have caused something or someone else to come out (which begs the question - who or what was Jephthah hoping or wanting to come out) is advocating for the overriding of human free will. If God were to go against or interfere with human free will, then it would effectively override the freedom of free will.

What lesson can we get from this? Jephthah made a rash proclamation instead of really leaning into what God would have asked of him - steady faith. A steadfast faith is the only condition that God gives for Him to step in on our behalf.

And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him.
Hebrews 11:6 (NIV)

There is no extra condition set to demonstrate our sincerity. God desires faith and action in faith. However, as this story showcases, making blind promises in our own flesh is a dangerous place to be in, and not what God asks of us. Ultimately, it is a life lived in accordance with the ways of God that we are called to, and we must be led of Him and not of our own will, lest we end up in a destructive place of our own choosing.