The use of the term Tartan in Isaiah 20 v 1 shows at least this section of Isaiah was written early, prior to 600 bc. Tartan is a military term in the Assyrian Army and is the highest position in the Army under the King himself. There would typically be one Tartan controlling the left side of the battlefield and another controlling the right side, and the King controlling the middle depolyment. The Assyrian Army ceased to exist when the Assyrian Empire ceased in 609 bc, when it was destroyed by the Babylonians. The language fell into decay and its military terms would have fallen more quickly into oblivion, seeing as there was no longer any Assyrian Army after 609 bc.
Furthermore, there is historical information in Isaiah chapters 1 to 39 which has been confirmed from other sources. Most cynics accept this section was written when it claims to have been written, soon after 700 bc.
Chapters 44 (late) and 45 (early in the chapter) refer to Cyrus, who was King of Persia and conquered Babylon and ended the Neo Babylonian Empire October 12th 539 bc.
In Isaiah it is written:-
Thus says the Lord, your Redeemer,
who formed you from the womb:
“I am the Lord, who made all things,
who alone stretched out the heavens,
who spread out the earth by myself, v24 ....
who says of Cyrus, ‘He is my shepherd,
and he shall fulfill all my purpose’;
saying of Jerusalem, ‘She shall be built,’
and of the temple, ‘Your foundation shall be laid.’” v28
Chapter 45 continues:-
Thus says the Lord to his anointed, to Cyrus,
whose right hand I have grasped,
to subdue nations before him
and to loose the belts of kings,
to open doors before him
that gates may not be closed:
2 “I will go before you
and level the exalted places,[a]
I will break in pieces the doors of bronze
and cut through the bars of iron,
3 I will give you the treasures of darkness
and the hoards in secret places,
that you may know that it is I, the Lord,
the God of Israel, who call you by your name.
4 For the sake of my servant Jacob,
and Israel my chosen,
I call you by your name,
I name you, though you do not know me.
5 I am the Lord, and there is no other,
besides me there is no God;
I equip you, though you do not know me,
Cynics claim this section must have been written once Cyrus has become King of Persia or around that time or afterwards, and that the Jews have dishonestly pretended this is prophecy when in fact it wasn't. So they claim chapters 40 to the end were written later than 1-39.
One problem with this is that the sins which are so vigorously condemned in 40-66 are the sins of the pre-captivity ie the sins before going into exile in Babylon, they are the sins of Israel and Judah prior to the destruction of the Northern tribes and Judah prior to the destruction of Judah in the early 500s bc. For instance, idolatry is condemned and the burning of their own children to Molech is condemned in chapter 57. These sins did not happen after the return from captivity 536 bc... they are exclusively the sins of the pre-captivity. After the captivity the Jews were exceedingly careful not to worship the idols of the surrounding nations.