Albert Barnes Commentary Isaiah 7:4

"and say unto him, Take heed, and be quiet; fear not, neither let thy heart be faint, because of these two tails of smoking firebrands, for the fierce anger of Rezin and Syria, and of the son of Remaliah." — Isaiah 7:4 (ASV)

Take heed – Hebrew ‘Keep yourself;’ that is, from fear.

Neither be fainthearted – Hebrew, ‘Let not your heart be tender;’ that is, let it not be easily moved; be strong, fearless.

For the tails ... – There is much beauty and force in this comparison. The design of Isaiah is to diminish the fear of Ahaz.

Instead, therefore, of calling them firebrands – burning and setting on fire everything in their way – he calls them the tails, that is, the ends, or remains of firebrand – almost consumed themselves, and harmless.

And instead of saying that they were burning and blazing, he says that they were merely smoking – the half-burned, decaying remains of what might have been once formidable.

The prophet also is just about to announce their approaching destruction by the Assyrians . He, therefore, speaks of them as already almost extinguished and incapable of doing extensive injury.

Son of Remaliah – Pekah (Isaiah 7:1). ‘It is by way of contempt that the king of Israel is not called by his own name. The Hebrews and Arabians, when they wish to speak reproachfully of anyone, omit his proper name and call him merely the son of this or that, especially when his father is but little known or respected. So Saul names David, in contempt, the son of Jesse; (1 Samuel 20:27, 1 Samuel 20:31).’ – “Hengstenberg.”