When faced with a sudden threat of war, the King of Israel responds not with immediate military action, but with a biting, mocking fable. This story is designed to humiliate the King of Judah and expose the massive gap in power and status between their two kingdoms.
The primary approach among commentators is that the fable serves to show deep contempt. The King of Israel compares himself to a massive, powerful cedar tree, while reducing the King of Judah to a lowly, worthless thorn bush [מצודת ציון, אברבנאל]. Because both plants happen to grow in the same region, the thorn makes the mistake of thinking they are equals and arrogantly proposes a marriage alliance [ביאור שטיינזלץ]. The underlying message is clear: even if the King of Judah had approached with a peaceful offer of marriage, the King of Israel would have viewed the connection as an absolute disgrace. Therefore, it is wildly absurd for the King of Judah to dare provoke a war [רד״ק, רלב״ג, מצודת דוד, אברבנאל].
This intense lack of respect is also fueled by history. The King of Judah is descended from the house of Ahab, a dynasty that the King of Israel's ancestors previously destroyed. Furthermore, the disaster waiting for the King of Judah is actually a punishment from God because he recently began worshiping the gods of Edom [רלב״ג].
In the story, a wild beast simply walks by and casually crushes the thorn [מצודת ציון]. This highlights how fragile the thorn really is compared to the solid cedar [ביאור שטיינזלץ]. In reality, the wild beast represents the massive army of Israel. The trampling serves as a blunt warning: because of such pride and arrogance, the Israelite forces will completely crush the King of Judah [רש״י, מצודת דוד].
Alongside this direct reading, another approach suggests that this fable was actually a common, well-known saying among the Israelites, based on the ancient history of Shechem and Dinah [מלבי״ם]. In this view, the thorn represents Shechem, or his father Hamor, who arrogantly approached the cedar, representing Jacob, to demand a marriage. The wild beast symbolizes Jacob's sons, Simeon and Levi, who ultimately wiped out the city. By using this specific story, the King of Israel delivers a dark threat: just as Shechem's arrogance brought total destruction upon his people, the King of Judah's pride will bring disaster upon himself and his entire kingdom [רש״י, מלבי״ם, חומת אנך].