John Gill Commentary 1 Samuel 15:2

"Thus saith Jehovah of hosts, I have marked that which Amalek did to Israel, how he set himself against him in the way, when he came up out of Egypt." — 1 Samuel 15:2 (ASV)

Thus saith the Lord of hosts
Of the celestial host of angels, and of the army of Israel, yea, of all the armies of the earth: this is premised to engage the attention of Saul:

I remember that which Amalek did to Israel ;
four hundred years ago:

how he laid wait for him in the way when he came up from Egypt ;
in the valley of Rephidim, just before they came to Mount Sinai, and fell upon the rear of them, and smote the feeble, and faint, and weary, see (Exodus 17:8) (Deuteronomy 25:17–19)