Some translations set each line of poetry farther to the right than the rest of the text to make it easier to read. The ULB does this with the poetry in 15:2, 5-21.
God told Jeremiah not to pray for the people because God was punishing them. This was meant to be an illustration for Jeremiah. Jeremiah was not to pray because the punishment was already determined. Yahweh also explains that even the great Moses could not help the sinful Jews. (See: [[rc://en/tw/dict/bible/kt/sin]])
Yahweh is going to use the Gentile nations to punish Judah. This would have been very offensive to the Jews. They would have also been surprised to learn that Yahweh was using the Gentile nations to punish his people. (See: [[rc://en/ta/man/jit/figs-explicit]])