It's amazing to me how often God shows himself to people, even those who are evil and set against Him. Despite Ahab's wicked ways, God defeated the Aramean army for Ahab so that he would know that He alone is God. Despite Ahab seeing miracle after miracle, despite God's direct and obvious help in battle, Ahab continued to live as he pleased, without God.
We have that same choice. God shows Himself to us over and over again. We can choose to be like Ahab, ignore God, and go our own way. But if we do, eventually disaster will strike- if not in this life, then for eternity. Or we can choose to follow Him. Things may not all go our way in this life, but we will live in victory with Him forever.
In this passage we see the beginnings of a mistake that Jehoshaphat made. He aligned himself with Ahab through the marriage of his son to King Ahab's daughter, and then made a military alliance with him. This ended absolutely disastrously.
As my Life Application Bible says....
When believers in leadership positions become allied with unbelievers, values can be compromised and spiritual awareness dulled. The Bible often warns against teaming up with unbelievers.
Who do we surround ourselves with? Obviously, we are to love unbelievers, but they are not to become our closest advisers, or we, too, risk being steered off course.