Albert Barnes Commentary 2 Kings 23:28

"Now the rest of the acts of Josiah, and all that he did, are they not written in the book of the chronicles of the kings of Judah?" — 2 Kings 23:28 (ASV)

Josiah lived for 13 years after the celebration of his great Passover. Of this period we know absolutely nothing, except that during this time he seems to have submitted to Nabopolassar, who, after the fall of Nineveh, was accepted as the legitimate successor of the Assyrian monarchs by all the nations of the western coast.

Josiah, after perhaps a little hesitation (see Jeremiah 2:18 and Jeremiah 2:36), followed the example of his neighbors and frankly accepted the position of an Assyro-Babylonian tributary. Matters remained this way until 608 B.C., when the great events happened which are narrated in 2 Kings 23:29.