New Defender's Study Bible Notes
2:39 another kingdom. The “breast and arms of silver” represented the two-sided kingdom of the Medes and Persians, which would dominate the known world from 538 B.C. to 334 B.C., when it was to be conquered by Alexander the Great, of Greece (see Daniel 8:20-21).
2:39 kingdom of brass. The “third kingdom of brass (or bronze),” was the great Greek empire, which continued until defeated by Rome, which became a world empire under Augustus in 31 B.C.