Nabopolassar had orders to proceed to Babylon, of which he was probably made viceroy, and to defend the southern capital against the rebels.

We may conclude that he obeyed these orders so far as to enter Babylon and install himself in office; but shortly afterwards he seems to have made up his mind to break faith with his sovereign, and aim at obtaining for himself an independent kingdom out of the ruins of the Assyrian power.

Having formed this resolve, his first step was to send an embassy to Cyaxares, and to propose terms of alliance, while at the same time he arranged a marriage between his own son, Nebuchadnezzar, and Amuhia, or Amyitis (for the name is written both ways), the daughter of the Median monarch.
Cyaxares gladly accepted the terms offered; the young persons were betrothed; and Nabopolassar immediately led, or sent, a contingent of troops to join the Medes, who took an active part in the great siege which resulted in the capture and destruction of the Assyrian capital.
A division of the Assyrian Empire between the allied monarchs followed.
While Cyaxares claimed for his own share Assyria Proper and the various countries dependent on Assyria towards the north and the north-west, Nabopolassar was rewarded by his timely defection, not merely by independence but by the transfer to his government of Susiana on the one hand and of the valley of the Euphrates, Syria, and Palestine on the other.

The transfer appears to have been effected quietly, the Babylonian yoke being peacefully accepted in lieu of the Assyrian without the necessity arising for any application of force.

Probably it appeared to the subjects of Assyria, who had been accustomed to a monarch holding his court alternately at Nineveh and at Babylon, that the new power was merely a continuation of the old, and the monarch a legitimate successor of the old line of Ninevite kings.
Of the reign of Nabopolassar the information which has come down to us is scanty.