Early in the morning on this day in 1862 soldiers of the Union stopped the Confederate offensive march north at the creek of Antietam in the fields of Maryland.

Soon we began to hear a most ominous sound which we had never before heard, except in the far distance at South Mountain, namely, the rattle of musketry. It had none of the deafening bluster of the cannonading so terrifying to new troops, but to those who had once experienced its effects, it was infinitely more to be dreaded. These volleys of musketry we were approaching sounded in the distance like the rapid pouring of shot upon a tinpan, or the tearing of heavy canvas, with slight pauses interspersed with single shots, or desultory shooting.

Nearly 100,000 men were ready to fight throughout the day. As the sun set only 77,000 were standing and 4,000 lay dead — the most casualties in one day in American history.

The majority of the Union effort was amassed at the center of the battlefield while smaller groups attacked first on the left, then center, and then the right. Their plan was to push in from a flank and only then drive forward with a numerical advantage. The initial attacks were mostly unsuccessful in making ground, however, and so the Union’s largest division never was fully engaged. The Union General was conservative and slow to react, despite having acquired a paper copy of the Confederate battle plans.

The Confederates then abandoned their offensive and retreated at night. This is believed to have been enough of an end to their march north that President Lincoln was able to issue the Emancipation Proclamation a few days later. Two months later, the Union General in charge at the Battle of Antietam was removed for failing to pursue the Confederates and win more decisively.

Update: Some interesting details in this video on how the battle set the stage for the President to renounce slavery