There is no doubt that Sumu-la-ilum was the real founder of Babylon's greatness as a military power. We have the testimony of his later descendant Samsu-iluna to the strategic importance of the fortresses he built to protect his country's extended frontier;[68] and, though Dûr-Zakar of Nippur is the only one the position of which can be approximately identified, we may assume that the majority of these lay along the east and the south sides of Akkad, where the greatest danger of invasion was to be anticipated. It does not seem that Nippur itself passed at this time under more than a temporary control by Babylon, and we may assume that, after his successful raid, Sumu-la-ilum was content to remain within the limits of Akkad, which he strengthened with his line of forts. In his later years he occupied the city of Barzi, and conducted some further military operations, details of which we have not recovered; but those were the last efforts on Babylon's part for more than a generation.
The pause in expansion gave Babylon the opportunity of husbanding her resources, after the first effort of conquest had been rendered permanent in its effect by Sumu-la-ilum. His two immediate successors, Zabum and Apil-Sin, occupied themselves with the internal administration of their kingdom and confined their military activities to keeping the frontier intact. Zabum indeed records a successful attack on Kazallu, no doubt necessitated by renewed aggression on that city's part; but his other most notable achievements were the fortification of Kâr-Shamash, and the construction of a canal or reservoir.[69] Equally uneventful was the reign of Apil-Sin, for though Dûr-muti, the wall of which he rebuilt, may have been acquired as the result of conquest, he too was mainly occupied with the consolidation and improvement of the territory already won. He strengthened the walls of Barzi and Babylon, cut two canals,[70] and rebuilt some of the great temples.[71] As a result of her peaceful development during this period the country was rendered capable of a still greater struggle, which was to free Sumer and Akkad from a foreign domination, and, by over-coming the invader, was to place Babylon for a time at the head of a more powerful and united empire than had yet been seen on the banks of the Euphrates.