‘In the year that the Tartan came to Ashdod, when Sargon the king of Assyria sent him, and he fought against Ashdod and took it, at that time Yahweh spoke by Isaiah the son of Amoz, saying, “Go and loose the sackcloth from off your loins, and put your shoe from off your foot.” And he did so, walking naked and barefoot.'
The Tartan (or turtanu) was the title given to the commander-in-chief of the armies of Assyria. When the rebellion took hold he was sent to subdue the rebels, and succeeded. Meanwhile Isaiah had been instructed by Yahweh to provide an acted out prophecy as a grim warning to Hezekiah and Judah of the folly of trusting in Egypt and her promises (which certainly failed on this occasion, and would continue to do so).
Isaiah was to loose the sackcloth that he wore round his loins, probably an indication of his prophetic status (compare 2 Kings 1:8), although it may have been in order to depict his deep mourning at the sins of the people, and also to take off his shoes. Then he was to walk barefoot and clothed only in an undergarment before the people as a stark reminder of the consequences of rebellion. Obediently he did as he was commanded. For three years the inhabitants of Jerusalem were constantly faced with the stark figure of the prophet in his strange garb, walking about the city, a constant warning to them of his message from Yahweh.
‘At that time.' That is, while everything was going on. His three year sign would be a reminder to all, while negotiations were going on both with Ashdod and with Egypt, that some dreadful end was being indicated, although all probably thought that it was to happen only to Ashdod, and to Judah if they took part.