See Rashi, who explains that Jeremiah accuses God, as it were, for being the source of the nations’ hatred towards us. Because the Torah forbids us to eat of their foods and marry their children, the nations tend to dislike us. This might be the idea behind the comment of our sages in Shabbos 88a, that Sinai is called such because the gentile’s hatred [sinnah] toward us came from Sinai. The passage is usually interpreted to mean that the gentiles are somehow jealous of us.
However, the verse seems to suggest the nations were happy specifically because God Himself caused our destruction. oweverHPerhaps then the meaning is as follows: Had
Thousands of years later, the Nazis too, would ask this question of the martyrs –“Where is your God now?” Rav Gifter (z’l) tells of what his great Rebbi responded moments before he was murdered, when his Nazi executioner mocked him by asking him this very question: “He is not only my God, He is your God also, and the whole world will yet find this out.”