•Why it earns a slot
Delivered at the precise moment the Union was fracturing, this address lays out Lincoln's constitutional case against secession and his strategy of firmness without provocation, making it the essential document for understanding how the United States entered the Civil War.
Delivered on March 4, 1861, as seven Southern states had already declared secession, Lincoln's address argues that the Union is constitutionally perpetual and that no state can lawfully leave it. He reassures the South that he has no intention of interfering with slavery where it exists, while firmly stating that he will enforce federal law and hold federal property. He closes with an appeal to shared memory and friendship, placing the responsibility for any conflict squarely on those who would choose to become aggressors.
This distillation is written from the freely available original, which is always the better read when you have the time: gutenberg.org.