Jehoshaphat followed the Lord, but was not wise. He did business with the kings of Israel. He joined them in battle. He thought of them a brothers. This opened the door to trouble. The house of David began to intermarry with the house of Ahab. In 2 Kings 8:16 Jehoram became king of Judah and married one of Ahab's daughters. Jehoram became an evil king. He should have know better, as Solomon followed the same path with terrible consequences.
So, God took away another piece of the kingdom from Judah. Edom successfully rebelled against Judah. Edom had been subject to Judah since David conquered them and set up garrisons in their country. (2 Samuel 8) Jehoram violated the covenant by marrying a foreign woman. She led him into evil. God continued to impose the penalties for breaking the covenant. (Leviticus 26)
The same principle is carried over into the N.T. 2 Cor. 6:14 says "Do not be yoked together wit unbelievers."It goes on through verse 18 giving examples. It applies to marriage and business relationships, any relationship where the parties are bound (yoked) together. I know many Christian women who married non-believing men because they were handsome, popular or good at sports. Then, the spent a life time of struggling to practice their faith and raise their children in the faith.
We should live this principle and teach it to our kids. Jehoshaphat did not. The whole nation suffered for it.