November 8 witnessed one of the most stunning upsets in U.S. political history when Donald Trump defeated Hillary Clinton. But it might not have been the most stunning upset. 150 years earlier, also on November 8, a U.S. presidential election produced a shocking winner in what may be the most consequential election ever held in America.

In the summer of 1864, the Civil War had been raging for three years. Already, well over eight hundred thousand Americans had been killed or wounded. Citizens in both the North and South were virulently sick of war and wanted the conflict ended. Events on the battlefield, just before the election that fall, would seal the South’s defeat, catapult Abraham Lincoln to victory and ultimately provide momentum for the passage of the Thirteenth Amendment, permanently freeing all slaves in America.

Though not recognized at that time, it was a key battle the summer before that actually dealt what would prove to be the fatal blow to the South. The fight took place in a farming village in southern Pennsylvania that few in America had ever heard of: Gettysburg.

Most people in America today, regardless of party affiliation, revere Abraham Lincoln and believe that he was equally and broadly popular in his time. Many would be shocked to learn that heading into the election of 1864, Lincoln was expected to lose. Less than three months before the election, the cofounder of the Republican Party and editor of the powerful New York Tribune, Horace Greeley, wrote to senior Republican leaders, “Mr. Lincoln is already beaten. He cannot be elected. And we must have another ticket to save us from utter overthrow.”

By all accounts, as late as August 1864, most experts expected former Union general George B. McClellan to handily defeat Lincoln. Yet barely two months before the election, Gen. William T. Sherman finally overpowered Confederate defenses and captured the city of Atlanta, sending a wave of euphoria throughout the Union. After the leaders of his own party had considered replacing him on the Republican ticket, Lincoln won the most lopsided victory in history when he defeated McClellan in the Electoral College, 212 to 21. Sherman’s victory at Atlanta—and hence Lincoln’s victory in 1864—was made possible by the Union victory at Gettysburg in July 1863.

Confederate commander Gen. Robert E. Lee launched an offensive into Union territory in the summer of 1863, in an attempt to relieve pressure on the South and possibly turn the support of Northerners against the war, leading potentially to a negotiated settlement. The Union troops, led at the time by Gen. George Meade, met the rebels near the outskirts of Gettysburg on July 1. On the first day of battle, Lee’s troops scored decisive victories. On the second day, again Union forces suffered setbacks, leading some to fear Meade’s troops might have to surrender the field and withdraw. The fate of the entire battle came down to the defense of one small hill on the Union’s far left flank, at an otherwise unremarkable hill known as the Little Round Top.

The defensive position was secured by the Twentieth Maine regiment, led by Col. Joshua Chamberlain, who had been a professor of modern languages at Bowdoin College before the war. If Lee’s troops could dislodge the Twentieth Maine from the hill, the rest of the Yankee line would be vulnerable, potentially leading to complete Confederate victory. Chamberlain was told to hold the hill at all costs; even if he lost every man, retreat was not permitted.

The Confederates likewise recognized the value of the hill and ordered five regiments from Alabama and Texas to take the hill. Vastly outnumbered, the Twentieth Maine stretched themselves to barely one deep. Southern troops recognized how thin the line was on the far end of the defenses, and ordered a flanking assault. Chamberlain moved some of his men as far left as he could and met the charge. The first rebel attempt was repelled. A second and third charge was likewise repulsed, but the last attack by the Texans and Alabamians had depleted most of the remaining ammunition among the federal troops.

Seeing that the Confederates were assembling for another charge up the hill, Chamberlain realized he didn’t have enough ammunition to survive one more assault. Remembering his orders to hold at all costs, the Colonel ordered his men to fix bayonets. When the enemy began its final attack, Chamberlain ordered his men to leave their protective positions at the top of the hill and to run headlong into the attacking Southerners.

The Twentieth Maine didn’t stand a chance. They were exhausted, most—including Colonel Chamberlain—were wounded, and many of the troops were out of ammunition. Yet despite these odds, miraculously, the Yankee troops dashed downhill, right into the teeth of the Southern attack, screaming and wildly flashing the steel of their bayonets, and took the Southerners by surprise. The audacity of the downhill assault shocked the attackers. Once the Union troops broke through the Southern line, a panic set in and the rebel troops abandoned the field. The Union flank held, and eventually the entire Confederate attack failed.

Many Americans today are familiar with the battle of Gettysburg. Most read about it in history books growing up. Some have even attended reenactments in the city itself. But it is very difficult to appreciate the savagery, the barbarity of the fight, or the sheer volume of carnage that afflicted both sides. While the battle itself is known by most Americans, few are aware of the aftermath of the fight. A description of what happened on the field of battle after the two armies moved on to continue the fight elsewhere may be most indicative of the horrific experience there.

The Civil War inflicted the most casualties on Americans of any war we’ve ever fought. The North and South suffered the greatest number of casualties in that war at the battle of Gettysburg. Over fifty-one thousand people—both soldiers and civilians—were killed or wounded that day. The cleanup of the aftermath was almost as horrific as the battle itself.

Most experts estimate there were well over three thousand total bodies left on the ground when both the Union and Confederate armies continued to the next fight. Neither side had the manpower to bury more than a few score of their men. You can imagine what would happen to the remains of that many human beings in the middle of the hot and desultory summer when temperatures were near one hundred.

Historian Gregory Coco, author of A Strange and Blighted Land: Gettysburg, The Aftermath of a Battle, exhaustively research what happened on the Gettysburg battlefield in the years following the fight. His descriptions are sometimes hard to read. At a speech given at Gettysburg at the book’s launch, he explained that there were thousands of dead soldiers, but also three thousand dead horses and two thousand other animals. “There was every type of corrupting, decomposing corpse you could imagine,” he said. The stench was horrifying for those tasked with cleaning up the land.

Burying that many people was a mammoth chore, leaving little room for honor or formalities. “There might have been as many as 25 to 30 burial trenches,” Coco explained, “and in these trenches were anywhere from 25 to 100 men. The way they would bury them would be to put the men in the trenches and then cover them with maybe four inches of soil.” This seemingly efficient method had unintended consequences.

After the quick burials in the shallow graves, heavy rains would expose the bodies again and you could see where “hands stuck out, feet stuck out, and skulls stuck out. The birds had finally come back after a few weeks and they began to peck at the bodies,” Coco continued. “But worse than that, there were wild hogs and dogs loose everywhere. They began to chew on the exposed body parts and actually pull them out of the ground. I don’t think there can be anything worse in a human’s mind than to see a human being—enemy or not—being eaten by a wild dog or hog.”

A Union surgeon at one battle site recorded that “stretched along, in one straight line, ready for interment, at least a thousand blackened bloated corpses with blood and gas protruding from every orifice, and maggots holding high carnival over their heads.” It is difficult to comprehend or reconcile on one’s mind that such staggering human suffering and destruction at the Battle of Gettysburg actually resulted in positive outcomes for the country.

Had Lee not been so operationally weakened by his defeat at Gettysburg, he would likely have been able to continue offensive operations and conceivably have brought Washington, DC, itself under assault; the entire course of the war would have taken radically different trajectory. However the war might have ultimately turned out, had Lee continued the offensive in the North beyond Gettysburg, it is likely that Atlanta would not have fallen by the summer of 1864 and Lincoln been defeated by McClellan. Without the moral force of Lincoln at the helm, the Thirteenth Amendment would most certainly not have been passed.

The tide of slavery was already waning in the world at the time of the Civil War, and it is a virtual certainty that the wretched institution would have ended at some point in American history, even without Lincoln. But at what cost? How many more years or decades would have passed before freedom was won for black Americans? Those answers are impossible to answer, but one thing is certain. As much difficulty as the country has with race relations today, they are nothing in comparison to what we would still be facing had freedom not come until sometime in the twentieth century.

Gettysburg was a major battle that inflicted grievous wounds on both the North and the South. Though even at the time the Union was recognized at having come out on top, their losses led to discouragement and discontent at the ambiguity of the outcome throughout the North. Yet the slight margin of victory proved to be just enough to allow Sherman’s troops to defeat the Confederacy in Atlanta the next year. One of the enduring lessons of Gettysburg is that one never knows how consequential events may prove to be in the future, even when the results are mixed and messy.

Daniel L. Davis is a retired U.S. Army colonel who served multiple tours in Afghanistan. He is a senior fellow with Defense Priorities. Follow him on Twitter @DanielLDavis1.

Image: View of Stones River National Cemetery over the barrel of an M1857 Napoleon 12-pounder smoothbore. Wikimedia Commons/Creative Commons//Hal Jespersen