John Gill 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 and rebel
The Targum is, "and do not receive my Word"; the Messiah, when come, neither his person, nor his doctrines and ordinances: you shall be devoured with the sword ;
of the Roman armies, as they were under Titus Vespasian; see (Matthew 22:7) (Luke 19:44) for the mouth of the Lord has spoken [it] ;
now, by Isaiah, as well as in former times, (Leviticus 26:25Leviticus 26:33) .