The fall of Ashdod under Assyria provides the historical setting for the sign-act.
Isaiah 20:1-6
1 In the year that Tartan came to Ashdod, when Sargon the king of Assyria sent him, and he fought against Ashdod and took it;
The prophet removes sackcloth and sandals, embodying the coming shame of Egypt and Cush.
2 at that time Yahweh spoke by Isaiah the son of Amoz, saying, “Go, and loosen the sackcloth from off your waist, and take your sandals from off your feet.” He did so, walking naked and barefoot.
As Isaiah went stripped and barefoot, so Egypt and Cush will be led away by Assyria in humiliation.
3 Yahweh said, “As my servant Isaiah has walked naked and barefoot three years for a sign and a wonder concerning Egypt and concerning Ethiopia,
4 so the king of Assyria will lead away the captives of Egypt and the exiles of Ethiopia, young and old, naked and barefoot, and with buttocks uncovered, to the shame of Egypt.
Those who relied on these nations for security are dismayed and ashamed.
5 They will be dismayed and confounded, because of Ethiopia their expectation, and of Egypt their glory.
The failure of Egypt and Cush raises the urgent question of true refuge.
6 The inhabitants of this coast land will say in that day, ‘Behold, this is our expectation, where we fled for help to be delivered from the king of Assyria. And we, how will we escape?’ ”