Moments before charging into battle, the King of Israel orchestrates a dramatic change of plans. Driven by a mix of military strategy and a deep fear of a terrible prophecy, he suggests that he and the King of Judah take on entirely different roles and appearances on the battlefield to help him escape his fate.
The primary approach among commentators is that the King of Israel decides to hide his identity. He plans to disguise himself as a common soldier while instructing Jehoshaphat, the King of Judah, to remain in his recognizable royal garments. This tactic is born from two distinct motives. On a practical, military level, the King of Israel knows that the Aramean army harbors a deep personal hatred for him and will actively hunt him down. At the same time, the enemy has no conflict with the King of Judah [רש״י, רד״ק, ביאור שטיינזלץ].
Beyond battlefield tactics, there is a profound spiritual anxiety at play. Although he publicly showed a lack of faith, the King of Israel is secretly terrified of the prophet Micaiah's warning of doom, which was directed solely at him. Because the prophecy did not target the King of Judah, he encourages Jehoshaphat to wear his royal robes without any fear of danger [רלב״ג, מצודת דוד]. Digging deeper into this spiritual motive, [מלבי״ם] explains that the King of Israel actually hopes to outsmart the divine decree. He believes that by dressing as an ordinary man and temporarily giving up his outward royal status, the prophecy of his downfall might be considered partially fulfilled, thereby canceling the actual death sentence hanging over him.
Presenting a completely different perspective, [אברבנאל] argues that the King of Israel is not planning a disguise at all. Instead, he is offering to take the lead in the fight. In this view, the King of Israel tells Jehoshaphat to rest safely behind the lines in his normal clothes, without armor or weapons, since he does not need to fight. Meanwhile, the King of Israel will put on his armor, head to the front lines, and actively search out the enemy.
Ultimately, all of these careful calculations and tricks prove useless, and God's plan prevails. The King of Israel is struck and killed by a randomly fired arrow, completely bypassing his careful disguise. On the other hand, Jehoshaphat stands out clearly in his royal clothing and is even mistakenly surrounded by Aramean commanders looking to capture a king. Yet, he is miraculously saved from harm because of his deep piety and honest heart [רד״ק, אברבנאל].