Albert Barnes Commentary 2 Chronicles 35:21

"But he sent ambassadors to him, saying, What have I to do with thee, thou king of Judah? [I come] not against thee this day, but against the house wherewith I have war; and God hath commanded me to make haste: forbear thee from [meddling with] God, who is with me, that he destroy thee not." — 2 Chronicles 35:21 (ASV)

The house with which I have war - Necho viewed Babylon as the successor and representative of Assyria—the hereditary enemy of Egypt—and he means that he is merely continuing an old hostility with which Josiah has nothing to do. No doubt the Assyrian and Egyptian armies had often passed up and down Syria by the coast route, without approaching Jerusalem or even touching the soil of Judea.

God commanded me to make haste: forbear thee from meddling with God - These are remarkable words in the mouth of a pagan; but ancient inscriptions show that the Egyptian kings, in a certain sense, acknowledged a single supreme god and considered their actions to be inspired by him (for example, the god Tum, whose name is comparable to the city Pithom in Exodus 1:11, was worshipped as Ankh, “the living One,” a title similar to “Yahweh”). Hence, Necho merely expressed himself as Egyptian kings were in the habit of doing.