Jamieson, Fausset and Brown on 2 Chronicles 35:21-22

21-22. But he sent ambassadors ... What have I to do with thee, thou king of Judah?--Not wishing to spend time, or strength in vain, Necho informed the king of Judah that he had no intention of molesting the Jews; that his expedition was directed solely against his old Assyrian enemy; and that he had undertaken it by an express commission from God. Commentators are not agreed whether it was really a divine commission given him through Jeremiah, or whether he merely used the name of God as an authority that Josiah would not refuse to obey. As he could not know the truth of Necho's declaration, Josiah did not sin in opposing him; or, if he sinned at all, it was a sin of ignorance. The engagement took place. Josiah was mortally wounded [2Ch 35:23].