The idiom "brought on them" means to cause to attack with his army. Alternate translation: "God caused the king of the Chaldeans to attack them, and he" (See: [[rc://en/ta/man/translate/figs-idiom]])
The king probably did not personally kill their young men. Rather, his army killed them. Alternate translation: "whose army killed their young men with swords" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Here "hand" is a metonym for the power to defeat them. Alternate translation: "God allowed the Chaldean army to defeat them" (See: [[rc://en/ta/man/translate/figs-metonymy]])