Jehoshaphat was a son of King Asa who ruled over Judah after his father died. During his reign, many important reforms were made, and later in his life he personally went about bringing people in Judah back to God.
Jehoshaphat fortified Judah against Israel fairly early in his reign, and adhered greatly to the Lord's commands. In only his third year in power, he sent officials and priests to teach the word of God to all in Judah. At the same time, it is noted that the fear of the Lord was around Judah, and rather than making war with him, the surrounding peoples brought tributes to him, but despite this Jehoshaphat continued to build up Judah's defenses and armies.
After making a marriage alliance with Ahab, he agreed to go on military expeditions and proceeded to help him against Samaria. After Ahab asks him to help him take Ramoth-Gilead, he agreed, but asked Ahab to seek the council of the Lord first. Doing so, Ahab brought together a massive council of "prophets" prophesying about how God will bring Ramoth-Gilead into "the king's" hand. Sensing something was off, Jehoshaphat asked if there might be one more prophet they could consult, and after bringing in the prophet Micaiah, found that there was indeed trouble waiting in battle for Ahab. Even though Ahab outright dismissed the prophet's testimony about how God and his angels plotted his doom at Ramoth-Gilead, Jehoshaphat made the mistake of going out with the king.
During the battle, Ahab, who disguised himself as an ordinary soldier, died from a "random" bow and arrow, while Jehoshaphat, who wore his royal clothes, returned in peace to his home. Upon returning, Jehu son of Hanani informed him that he has offended the Lord by allying with Ahab, but God still finds "some good" in him from the times he worked to bring the people back to the Lord.
After warring, he appointed judges over Judah and he strictly instructed them to warn people of what wrongs are brought to them in disputed cases brought to them. Soon after, individuals from afar came to attack him, and in fear he sought out the Lord and asks God to "execute judgement" upon them. As a result, the Lord deceives the invading armies into killing each other, and leaves Judah with an incredible amount of loot from the dead soldiers.
Near the end of his life, he joins forces with another Israel king, Ahaziah, who was also wicked, like his father Ahab. After building some ships to sail and take Tarshish, a prophet makes the prophesy that the Lord will destroy what he has made. This is not only fulfilled in the wrecking of the ships built by the two, but also in Jehoshaphat's son Jehoram's destructive actions during his reign.