I've been neglecting my Civil War anniversaries - though that's going to change around the beginning of May. Winters in those days usually meant everyone went into camp and tried not to die of dysentery and pneumonia, and went back to shooting one another in the spring... And that's what happened 150 years ago - Nathan Bedford Forrest set out on a raid, and on April 12, attacked Fort Pillow, in western Tennessee, on the Mississippi river. A vigorous fight resulted - the Union troops were outnumbers badly, and in a bad position, though they were entrenched - after a few hours of fighting, Forrest demanded their surrender, they refused, and the rebels charged, broke through - and the battle turned into a massacre.
The reason it did, and the reason it became such a touch point afterwards, is that most of the men killed were U.S. Colored Troops. (Though it didn't help, probably, that the white Union troops at Ft. Pillow were Tennesseans.) It was not unique - the Confederates treated Colored Troops appallingly - sometimes murdering them in cold blood, usually selling the prisoners as slaves. The massacre brought all this into the center of attention - leading, for instance, to the breakdown of arrangements for exchange and parole of POWS. The north demanded that captured black soldiers be treated the same as any Federal POW - the south refused. So during 1864, prisoner exchanges ended, and prison camps such as Andersonville became horrific death zones. The war became exceedingly hard in 1864 - Ft. Pillow contributed to that, directly, and as part of the larger changes marked by the use of Colored Troops. It was becoming a different kind of war - talk about a war to free the slaves became less talk, and more reality - blacks fought directly for their freedom; and Northerners increasingly accepted their cause as identical to the cause of the Union.