Charles Ellicott Commentary Isaiah 45:3

"and I will give thee the treasures of darkness, and hidden riches of secret places, that thou mayest know that it is I, Jehovah, who call thee by thy name, even the God of Israel." — Isaiah 45:3 (ASV)

The treasures of darkness ... —The heaped-up wealth of “gold-abounding” Babylon. The capture of Sardis, with all the riches of Crœsus, must have been almost as fruitful in plunder. (Herod. i. 84). The conqueror was to see in his victories the token of the protection of Jehovah, and so accept his vocation as the redeemer of His people.