2 Samuel 12:26

Joab … took the royal city. As the siege of Rabbah would be conducted by the slow process of blockade, it might easily be prolonged into the second year, and so give ample space for David's sin and its punishment by the death of the child. But more probably the narrator, having commenced the history of David's sin, completes the story before returning to his account of the war. Thus the capture of Rabbah would occupy some of the interval between David's adultery and Nathan's visit of rebuke, and would lessen the difficulty, which we cannot help feeling, of David remaining for nine or ten months with the guilt of adultery and murder resting upon him, and no open act of repentance. Some short time, then, after Uriah's death, Joab captured "the city of waters." This is not a poetical name for Rabbah, but means the "water city," that is, the town upon the Jabbok, whence the supply of water was obtained. The citadel, which occupied a high rock on the northwestern side, must, therefore, soon be starved into submission, and the whole of "the royal city," that is, of the metropolis of the Ammonites, be in Joab's power. He therefore urges David to come in person, both that the honour of the conquest may be his, and also because probably the blockading force had been reduced to as small a body of men as was safe, and the presence of a large army was necessary for completing the subjugation of the country, which would follow upon the capture of the capital.