When the Western tribes saw the replica of the altar on the other side of the Jordan, they were indignant. They interpreted the altar as a breach of faith. The Israelites were not supposed to offer sacrifices anywhere else but where the ark, the presence of God, was. They were preparing to do war against the Eastern tribes. When given an opportunity to explain their actions, the Eastern tribes declare the altar a "Witness" of their faith between them and not a functional altar to offer sacrifices.
A Witness gives a testimony, evidence, proof of something. The Eastern tribes wanted to make sure that their faith was not questioned because of their living on the other side of the Jordan. Am I willing to offer a testimony of my faith publicly? What am I willing to do to offer my testimony to those who live around me? I am called to be a witness to the world!
I admire the readiness to wage war against a breach of faith. The Israelites had tasted the wrath of God as a consequence to their breaches of faith and they were not willing to go through that again. Am I ready to wage war against breaches of faith? I can't forget that God has equipped me to wage war against sin, the flesh, and the world. But I must pick up the full armor of God. I must be willing to prepare for an encounter at any moment.
Lord, may I never be ashamed of my faith. May I find ways to be a witness to those around me. Give me the power to fight against sin every day of my life and dread a breach of faith with all my heart and soul. Amen