The reign of Jeroboam, King of Israel, was defined by massive military successes and the expansion of his kingdom's borders. Yet, a summary of his achievements reveals a highly unusual political and military move: a king of Israel conquering territories only to transfer them to a neighboring, competing kingdom.
Jeroboam captured the cities of Damascus and Hamath and gave them to the Kingdom of Judah. The primary approach among commentators is that this action is considered a restoration because these territories historically belonged to Judah. During the era of King David, Aram was conquered. David placed governors in Damascus and Hamath, subjugating their inhabitants. Over the years, Aram rebelled and reclaimed these cities from the kings of Judah. By conquering them once again, Jeroboam effectively returned the cities to their historical, rightful owners.
A complication arises, however, regarding the fact that this return to Judah was somehow accomplished within the context of Israel. Commentators offer two main ways to understand this dynamic. One approach focuses on the military reality. The restoration of these cities to Judah was achieved strictly through the strength and military might of the Kingdom of Israel's warriors, rather than by the army of Judah [מצודת דוד, אברבנאל, ביאור שטיינזלץ]. The King of Israel did not mind officially handing the territories over to Judah, as he viewed the lands as ultimately returning to the hands of the broader Israelite nation [רד״ק].
Another approach offers a political explanation for this arrangement. During this period, following earlier military victories by the kings of Israel, the entire Kingdom of Judah was actually conquered and subjugated under the rule of the Kingdom of Israel. Therefore, when Jeroboam conquered Aram, he could administratively assign Damascus and Hamath to Judah. Because Judah itself was entirely under Israelite control, these newly conquered cities effectively remained under Jeroboam's ultimate sovereignty [מלבי״ם].