Jehoshaphat (meaning Jehovah-judged) was the son and successor of Asa, king of Judah. Albright has dated his reign to 873 - 849 BC, while Thiele offers the dates 870 - 848 BC.
Jehoshaphat spent the first years fortifying his kingdom against Israel (2 Chronicles 17:1, 2), and destroying the idols of Baal in the land (1 Kings 22:43). In the third year of his reign he sent out priests and Levites over the land to instruct the people in the law (2 Chr. 17:7-9). The author of 2 Chronicles generally praises his reign, stating that the kingdom enjoyed a great measure of peace and prosperity, the blessing of God resting on the people "in their basket and their store."
Jehosaphat also pursued alliances with his contemporaries ruling the northern kingdom, the first being with Ahab, which was based on marriage. This alliance led to much disgrace, and brought disaster on his kingdom (1 Kings 22:1-33) with the Battle of Ramoth-Gilead. While Jehoshaphat safely returned from this battle, he was comfronted by the prophet Jehu, son of Hanni, (2 Chr. 19:1-3) who reproached him for this alliance. We are told that Jehoshaphat repented, and returned to his former course of opposition to all idolatry, and promoting the worship of Yahweh and in the government of his people (2 Chr. 19:4-11).
Again he entered into an alliance with Ahaziah, the king of Israel, for the purpose of carrying on maritime commerce with Ophir. But the fleet that was then equipped at Ezion-gaber was immediately wrecked. A new fleet was fitted out without the co-operation of the king of Israel, and although it was successful, the trade was not prosecuted (2 Chr. 20:35-37; 1 Kings 22:48-49).
He subsequently joined Jehoram, king of Israel, in a war against the Moabites, who were under tribute to Israel. This war was successful. The Moabites were subdued; but seeing Mesha's act of offering his own son in a human sacrifice on the walls of Kir-haresheth filled Jehoshaphat with horror, and he withdrew and returned to his own land (2 Kings 3:4-27).
The last notable event occured when the Moabites formed a great and powerful confederacy with the surrounding nations, and marched against Jehoshaphat (2 Chr. 20). The allied forces were encamped at Engedi. The king and his people were filled with alarm, and betook themselves to God in prayer. The king prayed in the court of the temple, "O our God, wilt thou not judge them? for we have no might against this great company that cometh against us." Amid the silence that followed, the voice of Jahaziel the Levite was heard announcing that on the morrow all this great host would be overthrown. So it was, for they quarrelled among themselves, and slew one another, leaving to the people of Judah only to gather the rich spoils of the slain. This was recognized as a great deliverance wrought for them by God (890 BC). Soon after this victory Jehoshaphat died after a reign of twenty-five years at the age of sixty, and was succeeded by his son Jehoram (1 Kings 22:50).
Initial text from Easton's Bible Dictionary, 1897 -- Please update as needed