What does Daniel 8:20 mean?

This begins an explanation of Daniel's vision (Daniel 8:1–2), given by the angel Gabriel (Daniel 8:16). One of the first images Daniel saw was a male sheep—a ram—with two slightly different horns (Daniel 8:3–4). This animal was powerful and ran unchecked. Gabriel explains that this depicts the Medo-Persian empire. In this context, the "kings" and the "kingdom" are effectively one and the same.

The two parts of this kingdom, Media and Persia, were always somewhat distinct. This is reflected in other prophetic images (Daniel 2:32; 7:5). Over time, the later-arising Persians overtook and assimilated the Medes. Eventually the name "Media" became irrelevant and the kingdom came to be known as "Persia." Persia made great conquests north, west, and south of its origins. Their armies were infamously massive. The empire eventually included Babylonia, Mesopotamia, Syria, Asia Minor, Colehis, Armenia, Iberia, Palestine, Ethiopia, Egypt, and Libya.