President Abraham Lincoln’s Emancipation Proclamation takes effect on Jan. 1, 1863. This landmark document comes as a divided nation nears its third year of grinding, grueling warfare.

The proclamation doesn’t end slavery outright, but it declares that those held as slaves in the states in rebellion against the Union “are, and henceforward shall be free.” The immediate effect is to make the nation’s Civil War far more than a fight to restore the Union, transforming the scope of the conflict as a key step toward the eventual demise of slavery.

The run-up to New Year’s Day 1863 brings no pause in the fighting 150 years ago in the Civil War. On Dec. 26, 1862, Union divisions approaching from the Yazoo River crash into Confederate forces in swampy terrain as they embark on an audacious bid to seize the rebel-held city of Vicksburg, Miss.

The fierce battle of Chickasaw Bayou, or Walnut Hills, erupts. When Union Maj. Gen. William T. Sherman orders his forces to advance, his units are thrust backward by the Confederate defenders. Sherman was hopeful of capturing Vicksburg outright, but the battle left heavy Union casualties and dashes any Union hopes of a swift victory.

The fighting, however, augurs the start of the Vicksburg campaign, which would be renewed by the Union months later in 1863.