Its a very grim reminder of the horrors of war. It took a lot to create the nation of America. It was very costly to also bring about the freedom of all people. America set a precedent for the rest of the world to follow.
A good rendition: https://youtu.be/N_lCmBvYMRs
> While questioning a suspected German spy, he performed a word association test on him. When Griswold said "terror of flight," the suspect replied, "gloom of the grave." This was evidence that he was a spy who had been trained up in Americanisms, since the two phrases allude to a line in the third verse of "The Star-Spangled Banner" and no native-born American could possibly be familiar with the third verse of the national anthem ("except for me, and I know everything," added Griswold).
I never put the context together to realize it was talking about threatening to return free people to slavery.
More Wikipedia context about these lines at https://en.wikipedia.org/wiki/The_Star-Spangled_Banner#slave . That page also mentions the NAACP call to remove the national anthem/
https://en.wikipedia.org/wiki/Deutschlandlied
"Germany, Germany above all, [...]"
[edit: this is only intended to point this out as a "similar curiosity"; both are quite questionable in current times.]
