Some translations set poetry farther to the right than the rest of the text to show that it is poetry. The ULB does this with the poetry in 46:3-12, 14-24.
Much of Jeremiah is concerned with Yahweh's punishment of Judah. Yahweh used Egypt to punish Judah. Because Yahweh is just, he is going to punish Egypt for their evil deeds as well. (See: [[rc://en/tw/dict/bible/kt/justice]] and [[rc://en/tw/dict/bible/kt/evil]] and [[rc://en/tw/dict/bible/kt/works]])
Jeremiah commands the Egyptian army to prepare for battle, even though they will not be able to defend themselves. Their punishment has already been determined by Yahweh.