For this is what the Lord says:
“I will extend peace to her like a river,
and the wealth of nations like a flooding stream;
you will nurse and be carried on her arm
and dandled on her knees.
As a mother comforts her child,
so will I comfort you;
and you will be comforted over Jerusalem.”
When you see this, your heart will rejoice
and you will flourish like grass;
the hand of the Lord will be made known to his servants,
but his fury will be shown to his foes.
See, the Lord is coming with fire,
and his chariots are like a whirlwind;
he will bring down his anger with fury,
and his rebuke with flames of fire.
For with fire and with his sword
the Lord will execute judgment on all people,
and many will be those slain by the Lord.
The promise is not merely for military victory. It includes being comforted in the same way a mother comforts a child. They will not just flourish like grass, but hearts will rejoice.
So what relation does this have to the Lord coming with fire and with his sword? I think it means that one does not reach this state easily. There is suffering that cannot be avoided. Hard work cannot be short circuited.