To understand why Sickles moved his corps out to the Emmitsburg Road at Gettysburg in July 1863, it is helpful to look back at his first battle as commander of the Third Corps that spring at the Battle of Chancellorsville.

Chancellorsville is famous for Stonewall Jackson's march around Joe Hooker's exposed right flank and subsequent mortal wounding that night while riding between the lines. But after Jackson's attack, Robert E. Lee's army was still divided in two and significantly outnumbered by Hooker's army. Holding the position in between the two halves of Lee's army was Sickles' Third Corps, on an open spot of high ground called Hazel Grove. But the position created a dangerous salient in the Federal line, and Hooker ordered Sickles to pull back to the main line, surrendering Hazel Grove to the Confederates. Lee took full advantage of this mistake, as Hazel Grove was a perfect artillery platform, and the Confederate guns were able to gain a rare advantage, blasting apart the main Federal position around Chancellorsville and rendering it untenable.

So now we flash forward to the morning of July 2, 1863. Hooker had been replaced by George Meade a few days before the battle, and Meade had no faith in Sickles' competence as an officer (as an aside, Sickles was one of the Civil War's political generals - men with essentially no military background appointed to high positions in the volunteer army to help bolster political support. Professional officers like Meade were dismissive of the value of these officers, while political officers were dismissive of the supposed superiority of the professionals).

Meade arrived on the battlefield on the night of July 1st after a meeting engagement that day had resulted in a significant Confederate tactical victory, but left the Federals in command of the high ground south of the town. Meade was initially focused on ways he might attack Lee's position, and not paying much attention to his left, which is where he ordered Sickles to set up his men. Sickles' orders were to position his men where Geary's division had been the night before, with their right flank connecting to the Second Corp's left flank on Cemetery Ridge. Geary's division had been positioned in the area of Little Round Top and north of it the night before, but Sickles seemed unable to figure this out. Sickles seems to never have even gone onto Little Round Top, and instead focused on the position directly in front of him.

The position directly in front of him was not a good one. The more defensible parts of Cemetery Ridge were occupied by the Second Corps north of him, his position was in lower, marshy ground. In front of him was a large woodlot, through which an attacking force could find cover until they were on top of his position, and where it was difficult to set up artillery to support his infantry. And most troubling to Sickles, about 3/4 of a mile in front of the position was a perfect artillery platform in a peach orchard on the Emmitsburg Road, which could be used by the Confederates to blast away at his position, just like at Chancellorsville.

Meade's staff found Sickles men to be in no position at all, camped in the woods behind Cemetery Ridge, while Sickles and his staff tried to figure out what to do. Sickles asked Meade if he could position his men as he saw fit, and Meade sent back word that of course he could, so long as his position connected with Hancock's corps and Little Round Top.

Sickles began to inspect the Peach Orchard position to the west of his line, and had the army's Chief of Artillery, Henry Hunt, come and inspect it with him. Hunt saw the advantages of the position, and later said that it probably was a better position for the army to take than on Cemetery Ridge, but he also saw the flaws in it - mainly that Sickles did not have enough men to man the entire position. Sickles asked if he could move to the position, and Hunt replied "not on my authority."

Soon, Federal sharpshooters engaged Confederate infantry in the woods west of the Peach Orchard, and Sickles began to fear that if he didn't take action he would lose his opportunity. Acting without orders, he marched his entire corps out to the Emmitsburg Road to cover the Peach Orchard, leaving his right flank half a mile in front of Hancock's left flank, his left flank ending in the Devil's Den west of Little Round Top, and with large gaps in the infantry of his line, some of it covered by unsupported artillery batteries.

At 3pm Meade called for a meeting of all his corps commanders, and received word that Sickles had moved his corps far out in front of the rest of the line. As Sickles arrived at Meade's headquarters, Meade told him to stay on his horse, and rode out with him to the Peach Orchard, sending off his Chief Engineer G.K. Warren to inspect Little Round Top as they rode south.

When they arrived Sickles argued his case - he had taken a commanding position, the ground he was on was higher than the ground he had been assigned. Meade pointed out that he would find even higher ground west of that, and so on until they reached the slopes of South Mountain, visible along the horizon behind the Confederate lines. Sickles asked if Meade wanted him to pull back to his original position, at which point, as if on cue, the Confederate artillery opened fire to prepare for Longstreet's assault on Sickles' line. Meade replied that he could try, but he didn't think that the Confederates would let him. As Meade rode away, he ordered his entire reserve corps (the Fifth Corps) to the left to support Sickles, and then ordered the entire Twelfth Corps off of Culp's Hill to support the left to (leaving only George Greene's brigade to hold off an entire Confederate division).

Longstreet's attack practically destroyed Sickles' Corps, capturing Devil's Den, the Wheatfield, the Peach Orchard and the whole of the Emmitsburg Road position. But Little Round Top held thanks to Warren, Strong Vincent and Patrick O'Rourke, and the army rallied on Cemetery Ridge - right where Sickles should have been in the first place.

As the Peach Orchard fell, Porter Alexander - the officer who had commanded the artillery on Hazel Grove - rushed his guns forward to the captured position, expecting to see the end of the war unfold before him as he reached the crest. Instead he saw the Army of the Potomac rallying in an even stronger position, with reserves coming from everywhere to meet the attack. Longstreet's hard fighting that day had won ground that gave the Confederate no particular advantage, and now they'd have to fight with fewer men, exhausted men, without many of its best officers. Lee had seen the Peach Orchard in much the same way Sickles had, and hoped to use the artillery platform to blast apart the Union line on the third day. But ultimately Meade was right in the end - Little Round Top and Cemetery Hill made the Peach Orchard neutral ground - a position from which no real advantage could be gained, and as the grand artillery battle the next afternoon went on, Alexander found that he could not drive the Federal guns or infantry away from the area south of Cemetery Hill where Lee's infantry assault was planned.

Sickles himself was nearly killed in the fighting on the 2nd, his right leg being shattered by a cannonball as he watched the action unfold. But his wounding gave him a political opportunity - within days he was back in Washington, DC, being visited by the President himself in his hospital bed, telling Lincoln and everyone who came to see him how his actions had prevented Meade from retreating from Gettysburg, how his forward position had blunted Longstreet's attack and saved Little Round Top and Cemetery Ridge. Meade meanwhile was busy pursuing Lee's retreating army, and unable to defend himself from Sickles' charges. For the rest of Meade's life, Meade never got full credit for winning the most important battle of the war, and for the rest of Sickles' life, Sickles was considered by the public to be the true hero of Gettysburg. It was not until after Sickles died in 1914 that historians began to reevaluate the battle and determine that, for whatever the merits of Sickles' desire to move out to the Emmitsburg Road, he had put the Army of the Potomac in a precarious position, and it was Meade's decision to rush reinforcements to the left that blunted Longstreet's attack and won the battle, not Sickles' insubordination.