When Zion sees her sons and daughters returning home in the arms of the nations, she is stunned (Isaiah 60:5). Her heart trembles (pachad; cf. Deuteronomy 28:66-67) and grows large (rachab; cf. Isaiah 54:2). In short, she has a heart attack, and a flushed face to prove it (“you will see and be radiant”).
Zion rejoices, but her joy is anything but frothy. It’s mixed with fear and awe at what Yahweh has done; it’s colored by amazement at the depth and wisdom of a God who redeems through suffering. She is in shock when she glimpses the pattern in her own history, and the history of the nations.
Zion experiences the dreadful joy that is the deepest joy.