Israel and Judah found themselves positioned between the great political and military powers of the time: Assyria and Babylon to the north (or east) and Egypt to the south. In Isaiah 18 and 19, the prophet deals with the power to the south. Cush is likely more or less synonymous with Egypt for this purpose. The region is described as one of unrest.
Ah, land of whirring wings that is beyond the rivers of Cush, (Isaiah 18:1 ESV)
The IVP Bible Background Commentary comments on Cush, and explains the difference between Cush and modern Ethiopia.
Cush can refer to several different places in the Old Testament, though it most frequently is the designation for the area translations usually render “Ethiopia.” This is misleading, for the area Cush refers to is not modern Ethiopia (Abyssinia), but the area along the Nile just south of Egypt, ancient Nubia (in modern Sudan). The boundary between Egypt and Nubia in ancient times was usually either at the first or second cataract of the Nile. It is unlikely that Nubia ever extended much beyond the sixth cataract at Khartoum.
The Theological Wordbook of the Old Testament comments on the term Cush (Ethiopia) as it is used in several Old Testament prophecies.
In several cases, especially in the prophets, Ethiopia is used in parallel construction as a synonym of Egypt (Isa 20:3-5; Ezek 30:4; Nah 3:9). This probably represents the dominance of Ethiopia (or, more precisely, Nubia) over Egypt between 750 and 663 B.C. Terhakah was a notable Nubian pharaoh who tried, unsuccessfully, to block Sennacherib’s westward expansion (2Kings 19:9 ; Isa 37:9). After 663 B. C. Egypt was independent of Nubia (Jer 46:9; Ezek 25:4, 5, 9).
Nubians now live in Sudan and southern Egypt near Aswan. Some of these people were displaced as a result of the building of the New High dam at Aswan and the formation of Lake Nasser. Some of the Nubians still live in Sudan. They are energetic people and good at selling their wares to the visiting tourists. At the first cataract of the Nile some Nubians have set up a small village with shops and camel rides for those brave enough to try. Notice the example below called Kush (Cush) House.
One sign reads “Welcome to Taharka Kingdom.” Taharka ((English Bible: Tirhakah). A photo showing Tirhakah under the protection of the god Amun is available here.