Isaiah now begins denounces Judah's trust in the arm of the flesh. These are "if ... then" clauses, except there is no conditional. Isaiah is describing actions that Judah has taken, and is linking those actions to their future consequences. Judah and the "far off nations" will act in vain when they believe in the arm of flesh rather than the power of the Lord.
Literary analysis: These predictions are bracketed by the O Immanuel of verse 8 and the "God is with us" of verse 10. As is typical of Isaiah, he is dealing in contrasts, and here the contrast is between the God who is still with them, and their choice to believe in man rather than their God.