Albert Barnes Commentary 2 Chronicles 30:6

"So the posts went with the letters from the king and his princes throughout all Israel and Judah, and according to the commandment of the king, saying, Ye children of Israel, turn again unto Jehovah, the God of Abraham, Isaac, and Israel, that he may return to the remnant that are escaped of you out of the hand of the kings of Assyria." — 2 Chronicles 30:6 (ASV)

The posts went - The bearers of the letters were probably the “runners” who formed a portion of the king’s bodyguard (2 Kings 10:25 note).

The kings of Assyria - Pul, Tiglath-pileser, and Shalmaneser may all be referred to in this passage (compare the cross-reference and 2 Kings 17:3). The passage by no means implies that the fall of Samaria and final captivity of the Israelites had yet taken place.