The power of Assyria being now (629-624 B.C.) greatly weakened, if not completely broken, Josiah aimed not merely at a religious reformation, but at a restoration of the kingdom to its ancient limits (see the 2-Kings 23:19 note).
With their mattocks - Or "in their desolate places" (compare Psalm 109:10). Another reading gives the sense, "he proved their house round about."

The cities of Manasseh - Even those who were under the government of the Israelitish king permitted their idols and places of idolatry to be hewn down and destroyed: after the truth was declared and acknowledged, the spade and the axe were employed to complete the reformation.

And so did he in the cities of Manasseh, and Ephraim, and Simeon, even unto Naphtali,.... Which though they belonged to the ten tribes, yet these being carried captive by the king of Assyria, they that were left became subject to the kings of Judah; see Gill on 2-Kings 23:19,
with their mattocks round about; or hammers or mauls, as Kimchi, or pick axes, such sort of instruments as were used in demolishing altars and images: the Targum is,"in the house of their desolation;''
and so other versions, "in their desolate places" (x), which were become such, the inhabitants being carried captive, and few left behind.
(x) "in desolatis locis suis, vel eorum", Montanus, Tigurine version, Rambachius.

with their mattocks--or, "in their deserts"--so that the verse will stand thus: "And so did [namely, break the altars and burn the bones of priests] he in the cities of Manasseh, and Ephraim, and Simeon, even unto Naphtali, in their deserted suburbs." The reader is apt to be surprised on finding that Josiah, whose hereditary possessions were confined to the kingdom of Judah, exercised as much authority among the tribes of Ephraim, Manasseh, Simeon, and others as far as Naphtali, as he did within his own dominion. Therefore, it is necessary to observe that, after the destruction of Samaria by Shalmaneser, the remnant that continued on the mountains of Israel maintained a close intercourse with Judah, and looked to the sovereigns of that kingdom as their natural protectors. Those kings acquired great influence over them, which Josiah exercised in removing every vestige of idolatry from the land. He could not have done this without the acquiescence of the people in the propriety of this proceeding, conscious that this was conformable to their ancient laws and institutions. The Assyrian kings, who were now masters of the country, might have been displeased at the liberties Josiah took beyond his own territories. But either they were not informed of his doings, or they did not trouble themselves about his religious proceedings, relating, as they would think, to the god of the land, especially as he did not attempt to seize upon any place or to disturb the allegiance of the people [CALMET].

2-Chronicles 34:6 and 2-Chronicles 34:7 form a connected sentence: And in the cities of Manasseh, in their ruins round about, there he pulled down the altars, etc. The tribe of Simeon is here, as in 2-Chronicles 15:9, reckoned among the tribes of the kingdom of Israel, because the Simeonites, although they belonged geographically to the kingdom of Judah, yet in religion remained attached to the worship on the high places practised by the ten tribes; see on 2-Chronicles 15:9. "And unto Naphtali" is added, to designate the kingdom of Israel in its whole extent to the northern frontier of Canaan. The form בתיהם בּחר (in the Keth. divided into two words) gives no suitable sense. R. Sal. explains, timentes in planitie habitare, sed fixerunt in monte domicilia, rendering it "in their mountain-dwellings." This the words cannot mean.
(Note: The lxx translate ἐν τοῖς τόποις αὐτῶν, expressing merely the בתיהם. The Targ. has צדיוּתהון בבית, in domo (s. loco) desolationis eorum.)
The Keri בּחרבתיהם, "with their swords," is suggested by Ezekiel 26:9, and is accepted by D. Kimchi, Abu Melech, and others, and understood to denote instruments with which the altars, groves, and images were cut down. But this interpretation also is certainly incorrect. The word is rather to be pointed בּחרבתיהם, in their wastes (ruins) (cf. Psalm 109:10), and to be taken as an explanatory apposition to בּערי: in the cities of Manasseh, namely, in their ruins round about; for the land had been deserted since the times of Shalmaneser, and its cities were in great part in ruins. The statement as to the locality precedes in the form of an absolute sentence, and that which is predicated of it follows in the form of an apodosis with ו consec. (וינתּץ). להדק כּתּת, he dashed to pieces to crush; the form הדק is not a perfect after ל, but an infinitive which has retained the vowel of the perfect; cf. Ew. 238, d.

Naphtali - Which was in the utmost borders of the kingdom of Israel. For it must be remembered, that the ten tribes were now gone into captivity; and those who were come in their stead were weak and few, and not able to withstand the power of Josiah.