דברי הימים א, פרק י״א, פסוק ד׳

Transitioning from a regional leader over the tribe of Judah to the king of an entire nation required David to establish a unifying capital and demonstrate undeniable military strength. Conquering the city of Jerusalem served both the political and psychological needs of his new reign.

David could no longer rule from his previous seat in Hebron. The city was located deep within the territory of Judah, physically distant from the other tribes and too closely associated with his own tribe [ביאור שטיינזלץ]. He needed a new center of government that could be shared by all the people. Because the tribes of Judah and Benjamin were now united under his rule, David set his sights on Jerusalem. The city straddled the border, sitting half in the territory of Judah and half in the territory of Benjamin, making it the ideal location for a national capital [מלבי״ם].

Beyond the political strategy, the timing of the attack carried critical importance. David and the people of Israel marched to war against Jerusalem immediately after he was crowned by the entire nation [מצודת דוד]. This swift action was necessary to prove his independent power. It was designed to silence any doubts among the people, preventing claims that his past military successes were merely the result of King Saul's good fortune and that, left on his own, the new king was afraid to fight [רש״י, ביאור שטיינזלץ].

At that time, the city was known as Jebus. It was occupied by the Jebusites, one of the Canaanite tribes living in the land [ביאור שטיינזלץ]. They had remained entrenched in the city for years because the tribes of Judah and Benjamin had been unable to drive them out during previous generations [מצודת דוד].