Assyrian expansion continued under Shalmaneser I's son Tukulti-Ninurta I (reigned 1243-1207 BC), who conquered further, exploited more and penetrated deeper. He started with the north, studying the Uqumeni kingdom (aka Uqumani, later Qumani), the principal among the Qutian federation, in anticipation of conquering the land. Upon his victory, he imprisoned the Uqumeni princes at Ashur until they took an oath of allegiance. This allegiance not only reduced Uqumeni to vassaldom, but also incurred a hefty annual tribute. This opened up a channel between Assyria and its vassal, and some Uqumeni names have been found amongst receipts for rations for Assyrian workers.
Next, Tukulti-NInurta I charged south to Babylonia. After many boundary clashes, Tukulti-Ninurta I initiated a full invasion into Babylonia that was immortalized in a laudatory Assyrian epic. Assyria's defeat of Babylonia was not like the defeat of northern barbarians, but rather a huge cultural coup equivalent to sacking Vatican City, Jerusalem or Mecca in modern times. To justify this, the Tukulti-Ninurta epic claimed Babylon's tutelary deity Marduk had disapproved of Kashtiliash's resistance to Assyria, thus deserting Babylonia and giving Tukulti-Ninurta I permission to invade Babylonia. This was symbolized by an image of Tukulti-Ninurta taking the god Marduk to Ashur, where Marduk was revered even after Babylonia recovered its independence.
After conquering Babylonia, Tukulti-Ninurta I moved the capital of Assyria from Ashur. His prestige had begun to wane, and rather than further displease Ashur's citizenry with higher taxes (for construction labor) or land takeover (for space), he chose to revolve his government around a site on the opposite side of the Tigris: Kar-Tukulti-Ninurta. When Babylonia's Nechuchadnezzer I (1125-1105 BC) conquered back Babylonia and began the second dynasty of Isin after just seven years of Assyrian rule, Tukulti-Ninurta I's claim that he had divine approval to take over Babylonia was shattered. After gross exaggerations failed to save his public image, his son (Ashur-nasir-pal) and the nobles of Ashur conspired against him. Tukulti-Ninurta I was removed from the throne, imprisoned in a building at Kar-Tukulti-Ninurta and assassinated. Tensions and conspiracy destabilized Assyria, leading to a decline in Assyrian power.