In the first days of July 1863, two great armies converged at the small town of Gettysburg, in southern Pennsylvania. Begun as a skirmish between Union cavalry and Confederate infantry scouting for supplies, the battle escalated into one of the bloodiest battles of the Civil War.

The Union’s eventual victory in the Battle of Gettysburg would give the North a major morale boost and put a definitive end to Confederate General Robert E. Lee’s bold plan to invade the North. Widely viewed as a key turning point in the war, the battle would take on even more importance later that year, when President Abraham Lincoln traveled to Gettysburg to dedicate the battlefield’s cemetery.

By June of 1863, having just led his Army of Northern Virginia to a stunning victory in the Battle of Chancellorsville, Lee was riding high. From this position of strength, he convinced Confederate leaders to approve a bold strategy of invading Pennsylvania, hoping to deal the Yankees a crushing defeat on their home turf.

“Lee says more than once that he believes his men would be invincible,” explains Jennifer Murray, a history professor at Oklahoma State University and the author of On A Great Battlefield: The Making, Management, and Memory of Gettysburg National Military Park, 1933-2013. A successful invasion of Union territory, the Confederate general hoped, would convince Northerners to abandon their support for Lincoln’s war effort in droves.

On June 28, with Lee’s army on the move in Pennsylvania, Lincoln removed Joseph Hooker as commander of the Army of the Potomac, replacing him with George G. Meade. This marked the third change of command seen by the Army of the Potomac in 1863.

“The Union soldiers are confident in themselves,” Murray says. “But they're a little more questionable about their leadership, and about this string of commanders coming in again and again.”

Along with the news of the command change, Lee soon learned that the Union troops were closer than he expected them to be. “[Lee’s] cavalry, led by J.E.B. Stuart, is out sort of joy riding, and not doing a really good job of bringing intelligence over to Lee,” Murray points out. Abandoning his plan to drive deeper into Pennsylvania, toward Harrisburg, Lee ordered his army to concentrate at Cashtown, a tiny town located about eight miles west of Gettysburg.

With nearly a dozen roads leading into and out of town, Gettysburg was a key destination for moving troops. On June 30, a few Confederate divisions headed there in search of shoes and other supplies, and encountered two brigades of Union cavalry.

Though the bulk of the Army of the Potomac was still in Maryland, fulfilling Lincoln’s orders to stay between Washington and the rebel army, the cavalry units were scouting ahead to find out intel about the enemy position. After initially pulling back to Cashtown, the Confederate soldiers decided to go back to Gettysburg the next day (July 1) and get the supplies they needed, even if it meant confronting the Union troops.

“The first shot of the battle is fired a little bit after 7:00 in the morning,” Murray says. “Neither Meade nor Lee look to Gettysburg on a map and say, we're going to fight there. It begins as an accident, and then it escalates.”

The first day of fighting appeared to be another Confederate victory, as the rebels drove their Yankee counterparts into retreat through the town of Gettysburg. But Union troops still held the high ground south of town, on Cemetery Ridge, which would prove crucial in the days to come.

On July 2, Lee sought to press his advantage, launching massive assaults on both sides of the Union line. The hesitance of his subordinate generals allowed more Union reinforcements to arrive, strengthening their defensive positions and enabling them to stall the rebel onslaught. With over 20,000 casualties, the second day at Gettysburg would stand as one of the war’s bloodiest days of fighting.

Lee tried again on July 3, believing his “invincible” army could triumph with just one more push. But the attack, by fewer than 15,000 Confederate soldiers led by George Pickett, was a “catastrophic failure,” says Murray, with nearly 5,600 rebel soldiers killed, wounded or captured. The following day, Lee began preparations to move his army south, with Meade in pursuit. Ultimately, with the Confederates dug in along the Potomac, Meade decided against an attack, giving Lee’s forces time to cross the river back into Virginia (and earning Lincoln’s ire).

Casualties were high on both sides at Gettysburg, but the Confederates undoubtedly suffered more lasting damage. In all, some 28,000 Confederate soldiers were killed, wounded or missing, comprising one-third of Lee’s entire army, and including several of Lee’s trusted subordinates. Though the Confederate general would go on to other military victories south of the Mason-Dixon line, he would never again lead an invasion of the North.

On the other hand, Gettysburg reinvigorated the Union war effort, especially when combined with Ulysses S. Grant’s near-simultaneous capture of Vicksburg in the war’s western theater.

“Three days after the battle, the headline of the Philadelphia Enquirer reads ‘Waterloo Eclipsed,’” says Murray. “Just days after it ended, Philadelphians and Northerners are thinking of the Battle of Gettysburg as comparable to the battle that defeated Napoleon and completely reshaped the geopolitical situation of western Europe.”

For Union troops, stopping Lee’s invasion, and defeating rebel troops on northern soil, provided a much-needed surge in morale that would sustain them into the next grueling phase of war.

But the clash took on even more significance in November 1863, when President Lincoln delivered the Gettysburg Address. In one famously brief speech, Lincoln consecrated the battlefield, honored the sacrifice of the soldiers who died there and redefined the war as a struggle not just for the Union, but for the nation.

As Lincoln said, “...that we here highly resolve that these dead shall not have died in vain—that this nation, under God, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth.”