Assyria will take Egypt and Ethiopia as Prisoners

"Then the LORD said, "Just as My servant Isaiah has walked naked and barefoot three years for a sign and a wonder against Egypt and Ethiopia, so shall the king of Assyria lead away the Egyptians as prisoners and the Ethiopians as captives, young and old, naked and barefoot, with their buttocks uncovered, to the shame of Egypt." Isaiah 20:3-4

COMMENTARY - ISAIAH 20:3-4
3. three years--Isaiah's symbolical action did not continue all this time, but at intervals, to keep it before the people's mind during that period. Rather, join "three years" with "sign," a three years' sign, that is, a sign that a three years' calamity would come on Egypt and Ethiopia, (Isa 8:18). This is the only instance of a strictly symbolical act performed by Isaiah. With later prophets, as Jeremiah and Ezekiel, such acts were common. In some cases they were performed, not literally, but only in prophetic vision.
wonder--rather, "omen"; conveying a threat as to the future.
upon--in reference to, against.

4. buttocks uncovered--captives are found represented thus on Egyptian monuments (Isa 47:2, 3; Na 3:5, 8, 9), where as here, Egypt and Ethiopia are mentioned as in alliance. JFB 

View Section - Isaiah 20:1-6