Awake, awake, arm of the Lord,
    clothe yourself with strength!
Awake, as in days gone by,
    as in generations of old.
Was it not you who cut Rahab to pieces,
    who pierced that monster through?
10 Was it not you who dried up the sea,
    the waters of the great deep,
who made a road in the depths of the sea
    so that the redeemed might cross over?
11 Those the Lord has rescued will return.
    They will enter Zion with singing;
    everlasting joy will crown their heads.
Gladness and joy will overtake them,
    and sorrow and sighing will flee away.

As the prophet prays here, he is not for one moment imagining that a ‘part’ of God has gone to sleep. This is essentially a prayer, an urgent request, that God will do again what He has done before. He has shown His power mightily in the past. ‘Do it again, Lord’ is the heartfelt cry. Here is how it reads in ‘The New Living Translation’:

Wake up, wake up, O Lord! Clothe yourself with strength!
    Flex your mighty right arm!
Rouse yourself as in the days of old
    when you slew Egypt, the dragon of the Nile.
10 Are you not the same today,
    the one who dried up the sea,
making a path of escape through the depths
    so that your people could cross over?
11 Those who have been ransomed by the Lord will return.
    They will enter Jerusalem singing,
    crowned with everlasting joy.
Sorrow and mourning will disappear,
    and they will be filled with joy and gladness.

Barry Webb writes regarding verse 11:

‘In the end there is only one people of God, the ransomed of the LORD, and when all God’s purposes for them have reached their goal they will all be together in one place – Zion, the city of God. They will enter it with singing, and joy will be their crown for ever (11).

As so often, Isaiah’s vision reaches far beyond the particulars of history to its end; beyond the return from Babylon to the consummation it foreshadowed. And he could hardly wait for the dawning of that final day. There were many obstacles in its way, but he was sure that the strong arm of the LORD had lost none of its ancient power (9).’ ‘Isaiah’, p.202.