After the loss of these cities and the death of Sîn-šar-iškun, Aššur-uballiṭ II rallied what remained of the Assyrian army at Harran where, bolstered by an alliance with Egypt, he ruled for three years.
Aššur-uballiṭ's attempt at retaking it in 609 BC was repulsed whereafter he is no longer mentioned in contemporary chronicles, signalling the end of the ancient Assyrian monarchy.
[8] One of the major reasons for its fall was Assyria's failure to efficiently solve the so-called "Babylonian problem", the near constant rebellions in its southern provinces, especially in the ancient and prestigious city of Babylon.
[12] In 614 BC, the city of Assur, once the capital and still the ideological and religious center of Assyria, was captured, plundered and sacked by the Medes under Cyaxares, allies of Nabopolassar.
[3] Aššur-uballiṭ is likely the same person as the unnamed crown prince (designated heir) attested in inscriptions from Nineveh dated to 626 and 623 BC.
The last king to be crowned at the temple of Ashur at Assur had been Sîn-šar-iškun and with the city's destruction in 614 BC, the traditional Assyrian coronation ritual was now impossible.
Instead, Aššur-uballiṭ's formal title was crown prince (mar šarri, literally meaning "son of the king"),[16] which can be inferred from preserved documents, such as the following portion of a legal document from the city Dur-Katlimmu: Whoever contests the agreement, [missing portion] shall be his legal adversaries; the covenant of the crown prince shall seek vengeance; he shall give ten minas of silver.28th day, month Tebetu, eponym year of Seʾ-ilaʾi.
[17]The portion of the document that is important in regards to Aššur-uballiṭ's status is the mention of the "covenant of the crown prince" (adê ša mar šarri).
[18] Inscriptions from this time also record the name of the final Commander-in-Chief of the Assyrian army, Nabû-mar-šarri-uṣur[n 2], which means "O Nabu, protect the crown prince!".
The meaning of the name, "Ashur has kept alive", suggests that Assyria's chief deity and its empire would be ultimately successful in their battle against their enemies.
[18] At the time Aššur-uballiṭ became the ruler of Assyria in 612 BC, his main objective would have been to retake the Assyrian heartland, including Assur and Nineveh.
[7] After the Babylonians had ruled Harran for three months, Aššur-uballiṭ and a large force of Egyptian soldiers attempted to retake the city, but this campaign failed disastrously.
[2][22] Beginning in July or June 609 BC, Aššur-uballiṭ's siege lasted for two months, until August or September, but he and the Egyptians retreated when Nabopolassar again led his army against them.
[22] The eventual fate of Aššur-uballiṭ is unknown[2] and his siege of Harran in 609 BC is the last time he, or the Assyrians in general, are mentioned in Babylonian records.
Rowton speculates that Aššur-uballiṭ could have lived until 606 BC,[2] but by then the Egyptian army is mentioned in Babylonian sources without any references to the Assyrians or their king.
Throughout the next century, Egypt and Babylon, brought into direct contact with each other through Assyria's fall, would frequently be at war with each other over control in the Fertile Crescent.