(for a bit of background on this post, look here).
We meet our two kings right at the beginning of the passage...
Joram son of Ahab became king of Israel in Samaria in the eighteenth year of Jehoshaphat king of Judah, and he reigned twelve years. He did evil in the eyes of the LORD, but not as his father and mother had done. He got rid of the sacred stone of Baal that his father had made. Nevertheless he clung to the sins of Jeroboam son of Nebat, which he had caused Israel to commit; he did not turn away from them.
Now Mesha king of Moab raised sheep, and he had to supply the king of Israel with a hundred thousand lambs and with the wool of a hundred thousand rams. But after Ahab died, the king of Moab rebelled against the king of Israel. So at that time King Joram set out from Samaria and mobilized all Israel. He also sent this message to Jehoshaphat king of Judah: "The king of Moab has rebelled against me. Will you go with me to fight against Moab?"
"I will go with you," he replied. "I am as you are, my people as your people, my horses as your horses."
2 Kings 3:1-7, NIV
A bit of background...
After Solomon's death in 931BC, the nation of Israel split into two. The northern (larger) "half", often confusingly called Israel was ruled from Tirzah by a series of short-lived dynasties, most of which were ended quickly by coups. The southern (smaller) "half" was ruled from Jerusalem by the dynasty of David, who had earlier ruled the whole nation, and whom God had promised he would be with. The two kingdoms, as one might expect, fought quite a lot.
Then, around 880BC, Omri came to the throne in the north, and was the first king to establish a proper dynasty there. He was a strong (and evidence suggests brutal) king, who moved the capital from Tirzah to his new city of Samaria - the northern kingdom was sometimes called "Samaria" after that. In a remarkable insult, the Bible hardly says anything about Omri, except that he was wicked and didn't follow God (1 Kings 16:21-28). But archaeology suggests that he was incredibly influential - in fact Assyria called Israel after Omri for another 150 years. Although Omri only reigned for 12 years, it seems that he became the dominant king among the small states in the area, with the others paying tribute to him. Omri was succeeded by his son Ahab, then Ahab's two sons Ahaziah (who didn't last long) and Joram, who is the first king we meet here. But Ahab had been a wicked king, and God promised to destroy him and all his descendants. So We have Joram, the powerful king but he's under God's curse.
Probably late in Omri's reign, the other king, Jehoshaphat came to the throne in Judah. He was a much more godly king than Omri (and Omri's descendants after him), but is often criticised by preachers for being weak. However, we are told that he was at peace with Israel (1 Kings 22:44) - the first southern king we're told that of. We also know that Jehoshaphat tended strongly to ally himself to the king of Israel, as he does here, even acting as double for him once in a highly dangerous situation (1 Kings 22:29-33). What is going on?
Given the political situation, the size of the kingdoms, and what we know of Omri and his power and influence I think it makes most sense to say that Jehoshaphat was essentially a vassal king to Omri and his descendants. Why else would he put himself literally in the line of fire for Ahab, as in 1 Kings 22? Why else would he say "my people are as your people, my horses as your horses"? Joram is clearly the dominant king of the big and powerful kingdom; Jehoshaphat is following him around and doing what he says (well, mostly).
We also get hints of this with Mesha king of Moab. Mesha was clearly another king who was a vassal of Ahab. Now he is rebelling, so Joram gets an army together to crush him. And who does he call on? His two loyal "friends", Judah and Edom.
So if this is the situation, why doesn't the author of Kings say so explicitly? I think it's fairly clear what's going on in the passage, especially in the light of 1 Kings 22. While the author of Kings is describing actual historical events, he isn't always describing them as a modern historian would. His main interest isn't to provide a detailed history for telling us exactly who did what, where, when, how and why. He is far more interested, for example, in the fact that Omri and his descendants ignored God than that they were powerful. He's reporting facts, but he's reporting them from one point of view. Actually, that's what all historians do - they put their own slant on history, but this guy does it blatantly. If you want a good example which doesn't need archaeology, try 2 Kings 17, where he spends most of the chapter giving a sermon on why Israel got conquered.]
So we've got two kings - Joram the powerful Omride king of Israel with a curse and Jehoshaphat the weak vassal king of Judah but with amazing promises. What do they do?
"By what route shall we attack?" he asked.
"Through the Desert of Edom," he answered.
So the king of Israel set out with the king of Judah and the king of Edom. After a roundabout march of seven days, the army had no more water for themselves or for the animals with them.
"What!" exclaimed the king of Israel. "Has the LORD called us three kings together only to hand us over to Moab?"
But Jehoshaphat asked, "Is there no prophet of the LORD here, that we may inquire of the LORD through him?"
An officer of the king of Israel answered, "Elisha son of Shaphat is here. He used to pour water on the hands of Elijah."
Jehoshaphat said, "The word of the LORD is with him." So the king of Israel and Jehoshaphat and the king of Edom went down to him.
2 Kings 3:8-12, NIV
We see here the difference in the attitude of the two kings to God. Joram tries blaming God for their own mistakes, Jehoshaphat wants to ask God via the prophet Elisha (who is a Good Thing).
Incidentally, it's worth noting that God's original judgement on Joram's father, Ahab, was a big drought, back in 1 Kings 17. Then, the drought ended in 1 Kings 18 after Elijah (Elisha's predecessor and mentor) had confronted all the prophets of Baal, demonstrated that Baal was powerless and God could do whatever he wanted and then had the prophets of Baal killed. The lack of water here might well be pointing back to that.
What does God say?
Elisha said to the king of Israel, "What do we have to do with each other? Go to the prophets of your father and the prophets of your mother."
"No," the king of Israel answered, "because it was the LORD who called us three kings together to hand us over to Moab."
Elisha said, "As surely as the LORD Almighty lives, whom I serve, if I did not have respect for the presence of Jehoshaphat king of Judah, I would not look at you or even notice you. But now bring me a harpist."
While the harpist was playing, the hand of the LORD came upon Elisha and he said, "This is what the LORD says: Make this valley full of ditches. For this is what the LORD says: You will see neither wind nor rain, yet this valley will be filled with water, and you, your cattle and your other animals will drink. This is an easy thing in the eyes of the LORD; he will also hand Moab over to you. You will overthrow every fortified city and every major town. You will cut down every good tree, stop up all the springs, and ruin every good field with stones."
2 Kings 3:13-19, NIV
What does God think of these two kings, the impressive and dominant Joram and the weak Jehoshaphat?
Elisha doesn't even recognise Joram by name. He wouldn't give him the time of day. Without Jehoshaphat, Joram would be stuck in the desert, and his diagnosis of the situation would be right. Moab would destroy them, his empire would fall. But God recognises Jehoshaphat, the weak king, the king of Judah, the descendant of David, the king with all the promises. God recognises him and helps him and those with him.
Who is the dominant king now?
But the implications of the passage are far better. You see, Jehoshaphat isn't the end of the kings of Judah. There is a far greater king - Jesus, the weak king, the king of Judah, the descendant of David, the king with all the promises. God recognises him and God helps him and those with him. However sinful we are, however much we have ignored God, however much we deserve to be under God's curse, God will help us if we stand with his king, Jesus.