The three-day battle fought at Gettysburg was not initially intended by either commander. It started when Heth (of Hill's corps) marched towards the town, intending to get shoes for his ill-shod Confederate soldiers from the Gettysburg factory. He ran into Buford with two brigades of cavalry and matters escalated from there. Two Union corps under Howard and Sickles arrived mid-morning and Heth's troops were badly cut up. More Confederate troops marched in and fighting increased. Early, marching down the Harrisburg road caught and routed Howard mid-afternoon and soon the whole Union force was in retreat. Several thousand were captured as they tried to funnel through the Gettysburg streets. Those who escaped capture rallied on Cemetery Hill, named for the cemetery on its summit.
Lee sent a message to Ewell on the left to attack the position if he thought there was a chance of success. This was the sort of message he was accustomed to send to Stonewall Jackson and it would undoubtedly have sent Jackson charging in to dislodge the badly shaken Union troops. But Jackson was two months in his grave and Ewell was uncertain and so did not attack. About 25,000 Confederate and 20,000 Union troops were involved on this first day, with casualties 8,000 and 9,000 respectively (half of the latter being prisoners). During the night the rest of Meade's army, with the exception of Sedgwick's corps arrived and took up position. Meade was disposed on a curved ridge in the approximate shape of an inverted fish-hook with two hills, Little and Big Round Top, at the "eye". Lee's plan of attack was to feint with Ewell at the barb and attack with Longstreet at the Little Round Top end.
Little Round Top commanded the entire Union position and it was temporarily left uncovered but Brigadier General Warren, the Union army's chief engineer, acted on his own initiative to cover the hill. He was just in time, and a nasty little regimental level fight ensued, but he held on. Longstreet had no great belief in the attack but he sent it in, from south to north: Hood at 4 pm, McLaws at 5 pm and Anderson at 6 pm. Apart from gobbling up a good proportion of two divisions unwisely posted in a forward salient the attack achieved little, which was unsurprising as by now it was 8 brigades attacking 22 in a solid defensive position. Sedgwick had arrived while the attack was in progress and was added to the defence.
At the other end of the line, Johnston and Early attacked at 7 pm and 8 pm respectively. They were not particularly successful either, although Early did manage to effect a lodgement on the point of the hook. By now Meade had his army together and had 80,000 men and 354 guns against Lee's 50,000 and 272.
The morning of the third day saw heavy fighting in the north were Ewell was lodged. The fight swayed to and fro but the Confederate troops were driven off by mid-morning. The main assault was planned for the Union centre. Lee planned to use Pickett's division (about 4,600 men) and Heth's (about the same number but badly cut up on the first day) with a few others to bring the total up to about 11,000. A heavy bombardment started about 1.00 pm which pounded the Union centre for over an hour and a half, but it caused only few casualties. At 2.30, the Union batteries ceased fire to make the Confederates think they were knocked out or out of ammunition and the ruse worked. The main attack came in. The Confederate army advanced in beautifully dressed lines across the three quarters of a mile of open ground. The Union artillery opened fire again and against the massed ranks it was impossible to miss. Some troops did succeed in reaching the Union position and even made a brief breakthrough but they were soon repulsed by the reserve. Barely half of those who set out returned, the attack having cost 5,000 casualties. Remarkably, Confederate morale was not completely shattered and the army dug in grimly, awaiting Meade's counter-attack. But Meade had just seen the results of attacking an entrenched enemy and he didn't feel like trying it from the opposite point of view. Lee was ultimately able to retreat unmolested.
The battle was one of the bloodiest of the war with a total of over 50,000 casualties, 23,000 Union and 28,000 Confederate. Inside the Gettysburg cemetery was a sign, which had apparently escaped notice: "All persons found using firearms in these grounds will be prosecuted with the utmost rigor of the law."