Albert Barnes Commentary Isaiah 1:20

"but if ye refuse and rebel, ye shall be devoured with the sword; for the mouth of Jehovah hath spoken it." — Isaiah 1:20 (ASV)

But if you refuse, you shall be devoured with the sword — Your enemies shall come in and lay waste the land.

This prediction was fulfilled as a consequence of their continued rebellion, when the land was desolated by Nebuchadnezzar, and the nation was carried captive to Babylon.

It illustrates a general principle of the divine government: if people persevere in rebelling against God, they shall be destroyed.

The word "devour" is applied to the sword as if it were insatiable for destruction. Whatever destroys may be figuratively said to devour (see the notes at Isaiah 34:5-6; Lamentations 2:3; Ezekiel 15:4; Joel 2:3; Revelation 11:5) — where fire is said to devour.

The mouth of the Lord — Yahweh Himself. This had been spoken by the mouth of the Lord and recorded (Leviticus 26:33):

And I will scatter you among the heathen,
And will draw out a sword after you;
And your land shall be desolate
And your cities waste.

On these points God proposed to reason; or rather, perhaps, these principles are regarded as reasonable or as commending themselves to people.

These are the great principles of the divine administration: if people obey God, they shall prosper; if not, they shall be punished. They commend themselves to people as just and true, and they are seen and illustrated everywhere.