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 aggrees to go on military expeditions and proceeds to help him against Samaria. After Ahab asks him to help him take Ramoth-Gilead, he agrees, but asks Ahab to seek the council of the Lord first. Doing so, Ahab brings together a massive council of "prophets" prophesysing about how God will bring Ramoth-Gilead into "the king's" hand. Sensing something was off, Jehoshaphat asks if there might be one more prophet they could consult, and after bringing in the prophet Micaiah, finds that there is indeed trouble waiting in battle for Ahab. Even though Ahab outright dismisses the prophet's testimony about how God and his angels plotted his doom at Ramoth-Gilead, Jehoshaphat makes the mistake of going out with the king.
During the battle, Ahab, who disguised himself as an ordinary soldier, dies from a "random" bow and arrow, while Jehoshaphat, who wore his royal clothes, returns in peace to his home. Upon returning, Jehu son of Hanani informs 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 appoints judges over Judah and he strictly instructs them to warn people of what wrongs are brought to them in disputed cases brought to them. Soon after, individuals from afar come to attack him, and in fear he seeks out the Lord and asks God to "execute judgement" upon them. As a result, the Lord decieves 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 Isreal 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 fullfilled in the wrecking of the ships built by the two, but also in Jehoshaphat's son Jehoram's destructive actions during his reign.