The Battle of Gettysburg, fought from July 1 to July 3, 1863, was the largest battle fought in North America and the turning point of the American Civil War. On July 3, Confederate General George Pickett's legendary charge against the Union center on Cemetery Ridge was repulsed catastrophically. Union artillery and infantry fire decimated the Confederate assault; survivors staggered back across the valley. Pickett's division, approximately 12,000 men at the start of the charge, suffered over 50 percent casualties in roughly one hour. By the evening of July 3, Confederate General Robert E. Lee, recognizing that the battle was lost, made the decision to retreat to Virginia. On July 4, 1863—the national holiday commemorating American independence and the day that news arrived of the Confederate surrender at Vicksburg—Lee's army began its withdrawal.

Total casualties at Gettysburg were staggering. Union forces under Major General George Meade suffered 23,055 casualties: 3,155 killed, 14,531 wounded, and 5,369 missing. Confederate forces under General Lee suffered approximately 28,000 casualties, with at least 3,900 killed. In three days of fighting, more than 50,000 men became casualties—killed, wounded, or missing. The percentage of casualties was particularly severe for Confederate forces, which could not replace losses; for the Union, large reserves of manpower could eventually replenish depleted regiments, but those replacements would take time to train and organize.

July 4, 1863, brought heavy rain and profound sorrow to Gettysburg. Lee's army, its supply train stretched along the roads toward the Potomac River, began the retreat that would extend over a week. The Potomac River, swollen by recent rains, became an obstacle. Lee's army could not cross immediately; the river was too deep and moving too rapidly for safe fording. Lee's forces were briefly trapped in Maryland, north of the Potomac, vulnerable to pursuit and destruction. General Meade, the Union commander, failed to pursue Lee aggressively. Lincoln would later express bitter disappointment at Meade's passive response, believing that an aggressive pursuit of Lee's trapped army could have ended the war. Lee, however, had prepared escape routes and found them open. By July 14, the Potomac had fallen enough for the army to cross, and Lee's battered force retreated into Virginia.

The immediate aftermath of the battle left 7,000 or more bodies lying on the battlefield. No formal burial operations began on July 4; those would come later, undertaken by the Quartermaster Corps, by local civilians, and—in part—by Confederate prisoner details. The stench of decomposing bodies in the July heat was reported by witnesses for weeks to come. Civil War medical records indicate that burial crews took days to identify and move bodies from the main fields of combat. Many bodies were moved to mass graves; individual graves were not dug for many of the dead. Estimates suggest that significant numbers of bodies may never have been formally identified or individually buried.

The medical situation was overwhelming. Approximately 21,000 wounded men required medical care. Jonathan Letterman, the Medical Director of the Army of the Potomac, had pioneered a revolutionary system of mobile field hospitals and organized ambulance corps that proved highly effective in casualty evacuation. Letterman's system involved approximately 14,000 medical personnel—surgeons, nurses, attendants, and ambulance drivers. This represented the largest field medical operation in North American history at that time. Churches, schools, public buildings, barns, and private homes throughout Gettysburg and the surrounding countryside were converted into hospitals. Wounded men lay on floors, hay, and whatever bedding could be improvised. Surgeons worked for 24 to 48 hours without rest, amputating limbs, extracting bullets, and providing emergency stabilization. Chloroform and ether were used for anesthesia, but supplies were limited. Many wounded men suffered through operations without anesthesia.

The risk of infection and gangrene was extreme. Antibiotics would not exist for nearly a century. Surgeons sterilized instruments in boiling water and tried to keep wounds clean, but post-operative infection killed as many or more men as the initial wounds. Infections became apparent within days; men who had survived surgery began to develop fever, and within a week to 10 days, sepsis and gangrene claimed many lives. Of the approximately 21,000 wounded at Gettysburg, roughly 3,000 to 4,000 would eventually die of wounds or disease.

The impact on the civilian population of Gettysburg was devastating. The town of approximately 2,400 people had been surrounded by armies and had witnessed three days of artillery bombardment. Houses had been damaged or destroyed. Civilian stores had been looted or requisitioned. The fields surrounding the town were now muddy, bloodstained, and filled with abandoned equipment, waste, and bodies. Civilian women and men worked alongside military medical personnel to care for the wounded. Residents shared food, clothing, and shelter with casualties on both sides.

July 4, 1863, brought news of another Union victory: the surrender of Vicksburg, Mississippi, on the same day. For six weeks, Union General Ulysses S. Grant had besieged Vicksburg, the last major Confederate stronghold on the Mississippi River. The Confederate garrison, under Lieutenant General John Pemberton, surrendered on July 4 with approximately 30,000 men and massive quantities of supplies and artillery. The fall of Vicksburg gave the Union complete control of the Mississippi River and symbolically represented the turning of the war's momentum. The combination of Gettysburg and Vicksburg on the same day—July 4, 1863—represented the beginning of the end of Confederate military capacity.

The battlefield would eventually become Gettysburg National Cemetery. On November 19, 1863, four months after the battle, President Abraham Lincoln delivered his brief address at the dedication of the cemetery, forever linking the battle to the cause of Union and the proposition that all men are created equal. But on July 4, 1863, no such meaning had yet been assigned. The meaning of the sacrifice would be constructed over years and decades. For now, the focus was on burying the dead, healing the wounded, and—for Lee—retreating to fight another day.