Turtledove, Harry. "Must and Shall"
Divergence: 1864 CE
What if: Lincoln was killed by a stray bullet during a visit to the front lines in 1864.
Summary: Hannibal Hamlin becomes president and promises a Reconstruction based on revenge. Eighty years later, federal security agents visit occupied New Orleans during an investigation of gun-running.
Comments: Analyzed in James Marten's "Do Not Embrace Them Too Hastily".
Published: In Asimov's Science Fiction, November 1995; Nebula Awards 32: SFWA'S Choice for the Best Science Fiction and Fantasy of the Year (ed. Jack Dann), HBJ 1998 (0151003068BUY, 0156005522BUY); Roads Not Taken: Tales of Alternate History (eds. Gardner Dozois and Stanley Schmidt), q.v.; and Counting Up, Counting Down, Ballantine/Del Rey 2002 (0345442261BUY), 2005 (0345477987BUY).
Awards: Nominee: 1996 Nebula for best novelette. Honorable mention: 1995 Sidewise Award for best short-form alternate history.