•Why it earns a slot
Why it earns a slot: it is the poem that tried to put music back into English verse, with printed stage directions for tempo and volume, and it names King Leopold's Congo atrocities in verse decades before most history books did.
Vachel Lindsay's 1914 collection turns poetry into a chant, opening with a drum-thumping vision of the Congo that swings from barroom horror to King Leopold's atrocities to a missionary's death, then closes with Lincoln's ghost pacing Springfield over a war just begun.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.