Today is the 150th anniversary of a document that profoundly changed the Union’s mission in the Civil War. The Preliminary Emancipation Proclamation — which declared Jan. 1, 1863 the date when African Americans held as slaves in any part of the Confederacy that continued the war effort would be considered “forever free” — may have been meant as an announcement of future action, but to many enslaved people it was an invitation to liberate themselves immediately.

But for all the lofty talk in Washington, the cold reality was that no plan was in place to care for the thousands of slaves who would run to the safety of Union military camps on Jan. 1, or any day before that. Thousands did indeed begin their trek to freedom well before the end of the year and arrived wearing tattered clothes and weak from hunger, quickly overwhelming any services the military could offer.

Historians have long focused on the joy of liberation, but little has been said about the hell that awaited many who turned to the Union military for refuge. Jim Downs in his new book, “Sick From Freedom: African-American Illness and Suffering During the Civil War and Reconstruction,” has changed that. Freedom is a glorious concept, but dying for lack of shelter, food and medical care is not.

This is a difficult book to read. Downs, an assistant professor of history at Connecticut College, has done his research well and includes personal stories of families who left not only slavery but the only homes they knew, only to die after reaching Union lines.

Downs quotes Matilda Hatchett, a former slave who was interviewed long after the war ended, as saying: “We was freed and went to a place that was full of people. We had to stay in a church with about twenty other people and two of the babies died there on account of exposure. Two of my aunts died, too, on account of exposure then.”

Of the estimated 500,000 refugees who fled slavery before the war ended, neglect killed many. Illness killed many more. Anticipating a short war, neither the North nor the South was prepared for the problems of a protracted war, particularly the care of wounded soldiers and the treatment of various diseases that were rampant in both camps and freedmen’s villages.

Downs has written a scholarly book about emancipation that should open a whole new discussion about how it was achieved. If there is any doubt about his assertions, he has included 56 pages of footnotes.