The wonders God wrought for the Israelites were to encourage them to act vigorously themselves. Thus the war against Satan's kingdom, carried on by preaching the gospel, was at first forwarded by miracles; but being fully proved to be of God, we are now left to the Divine grace in the usual course, in the use of the sword of the Spirit.
God encouraged Joshua. Fresh dangers and difficulties make it necessary to seek fresh supports from the word of God, which we have nigh unto us for use in every time of need. God proportions our trials to our strength, and our strength to our trials.
Joshua's obedience in destroying the horses and chariots, shows his self-denial in compliance with God's command. The possession of things on which the carnal heart is prone to depend, is hurtful to the life of faith, and the walk with God; therefore it is better to be without worldly advantages, than to have the soul endangered by them.