This is a question about Assur-Danin-Pal and his relationship to the Babylonian kings. First I need to introduce the characters:
Shalmaneser III: King of Assyria (859-824)
Assur-Danin-Pal: His son, who rebelled
Shamsi-Adad V: his other son, who put down the rebellion and became king in 824
Marduk-Zakir-Sumi I: King of Babylon (855-819), contemporary of Shalmaneser III
Marduk-balāssu-iqbi: his son (r. 819-813)
Marduk Zakir Sumi I helped Shamsi Adad V put down the rebellion. Shamsi Adad V was therefore indebted to the Babylonian King and married his daughter Shammuramat. This is conventional knowledge.
The page for Assur Danin Pal says that he (the usurper) sought the support of the Babylonian king Marduk-balāssu-iqbi. It describes the revolt as "marauding armies of Babylonians". This information is uncited and seems obscure. Marduk-balassu-iqbi suceeded his father a few years later.
Marduk-balassu-iqbi initially ruled an area in northeast Babylon that had been contested for centuries between Assyria and Babylon. He would have been well positioned to invade Assyria. If the above account is true then it raises the question of what the relationship was between the Babylonian king and his son. Was he acting as a vassal or rebel to his father when he backed Assur-Danin-Pal? It seems unlikely that his father would let him act on his own from this strategic location, unless he wanted him to embark on a fool's errand. The chronology may be telling; the son led his armies into Assyria's civil wars then his father had the last word. The son later became a rival to his sister Shammuramat.
Bounty: Did Marduk-ballasu-iqbi help Assur-Danin-Pal?
Can this also be answered: What was the relationship of Marduk-Zakir-Sumi to his son's invasion, if it did happen?