Albert Barnes Commentary Jeremiah 51:11

"Make sharp the arrows; hold firm the shields: Jehovah hath stirred up the spirit of the kings of the Medes; because his purpose is against Babylon, to destroy it: for it is the vengeance of Jehovah, the vengeance of his temple." — Jeremiah 51:11 (ASV)

Make bright - Rather, Sharpen.

The Medes (Genesis 10:2) were a branch of the great Indo-European family, who as conquerors had seized the vast regions extending from the Caspian Sea to the eastern borders of Mesopotamia, but without being able to dispossess the Turanian tribes who had previously lived there. They were divided into numerous clans, each with its own local chief, the leaders of the larger sections being those who are here called kings.