The Chaldeans remained subjugated and quiet during this period, and the next major revolt in Babylon against the Assyrian empire was fermented not by a Chaldean, Babylonian or Elamite, but by Shamash-shum-ukin, who was an Assyrian king of Babylon, and elder brother of Ashurbanipal (668-627 BCE), the new ruler of the Neo-Assyrian Empire.
Shamash-shum-ukin (668–648 BCE) had become infused with Babylonian nationalism after sixteen years peacefully subject to his brother, and despite being Assyrian himself, declared that the city of Babylon and not Nineveh or Ashur should be the seat of the empire.
Though Assyria had more than a hundred kings throughout its long history, Sennacherib (alongside his son Esarhaddon and grandsons Ashurbanipal and Shamash-shum-ukin) is one of the few kings who were remembered and figured in Aramaic and Syriac folklore long after the kingdom had fallen.
Upon his death, and in an effort to maintain harmony within his vast empire (which stretched from the Caucasus to Egypt and Nubia and from Cyprus to Iran), he installed his eldest son Shamash-shum-ukin as a subject king in Babylon, and his youngest, the highly educated Ashurbanipal (669–627 BC), in the more senior position as king of Assyria and overlord of Shamash-shum-ukin.
Despite being an Assyrian himself, Shamash-shum-ukin, after decades subject to his brother Ashurbanipal, declared that the city of Babylon (and not the Assyrian city of Nineveh) should be the seat of the immense empire.
As late as the accession of Assur-bani-pal and Shamash-shum-ukin, we find the Babylonians appending to their city laws that groups of aliens to the number of twenty at a time were free to enter the city;
The Uruk King List (IM 65066, also known as King List 5), a record of rulers of Babylon from Shamash-shum-ukin (r. 668–648 BC) to the Seleucid king Seleucus II Callinicus (r. 246–225 BC), accords Neriglissar a reign of three years and eight months, consistent with the possibility that Neriglissar died in April.
He quickly defeated his brothers in 681, completed ambitious and large-scale building projects in both Assyria and Babylonia, successfully campaigned in Media, the Arabian Peninsula, Anatolia, the Caucasus, and the Levant, defeated and conquered Egypt, and ensured a peaceful transition of power to his two heirs Ashurbanipal and Shamash-shum-ukin after his death.
his eldest living son Shamash-shum-ukin was selected as the heir to Babylon whilst a younger son, Ashurbanipal, was selected as the heir to Assyria.
While Ashurbanipal's mother was likely Assyrian in origin, Shamash-shum-ukin was the son of a woman from Babylon (though this is uncertain, Ashurbanipal and Shamash-shum-ukin may have shared the same mother) which would probably have had problematic consequences if Shamash-shum-ukin was to ascend to the Assyrian throne.
Esarhaddon probably surmised that the Babylonians would be content with someone of Babylonian heritage as their king and as such set Shamash-shum-ukin to inherit Babylon and the southern parts of his empire instead.
It is clear that Ashurbanipal was the primary heir to the empire and that Shamash-shum-ukin was to swear him an oath of allegiance but other parts also specify that Ashurbanipal was not to interfere in Shamash-shum-ukin's affairs which indicates a more equal standing.
In order to ensure the succession of Ashurbanipal and Shamash-shum-ukin, Esarhaddon himself also concluded succession treaties with at least six independent rulers in the east and with several of his own governors outside the Assyrian heartland in 672.
When Esarhaddon made his subjects swear to uphold his wishes in regards to the succession of Ashurbanipal and Shamash-shum-ukin, some of the vassals made to swear allegiance to his successors were rulers and princes from Media.
After attending his brother's coronation, Shamash-shum-ukin returned the stolen statue of Bel to Babylon and became the king of Babylon.
Despite his royal title, Shamash-shum-ukin was a vassal to Ashurbanipal;
Most of Shamash-shum-ukin's early reign in Babylon was spent peacefully, restoring fortications and temples.
As Shamash-shum-ukin grew stronger, he became increasingly interested in becoming independent of his brother.
In 652 Shamash-shum-ukin allied with a coalition of Assyria's enemies, including Elam, Kush and the Chaldeans, and forbade Ashurbanipal from any further sacrifices in any southern city.
By 650 Shamash-shum-ukin's situation looked grim, with Ashubanipal's forces having besieged Sippar, Borsippa, Kutha and Babylon itself.