On the third day of the battle (July 3, 1863), Confederate General Robert E. Lee ordered an attack on the Union center, located on Cemetery Ridge. This offensive maneuver called for almost 12,500 men to march over 1,000 yards of dangerously open terrain.
Preceded by a massive but mostly ineffective Confederate artillery barrage, the march across open fields towards the Union lines became known as Pickett's Charge; Maj. Gen. George E. Pickett was one of three division commanders under the command of Lt. Gen. James Longstreet, but his name has been popularly associated with the assault. Union guns and infantry on Cemetery Ridge opened fire on the advancing men, decimating the Confederate ranks. Pickett's men were able to breach the Union lines in just one place, a bend in the wall that has become known as "the Angle." This gap in the Union line was quickly closed with any Confederate soldiers who had breached it being quickly captured or killed.
Lee's Army of Northern Virginia retreated the next day, leaving Gettysburg for Virginia. Even though the war lasted almost another two years, Lee launched few offensive operations during that time, none of them near the scale of the Gettysburg Campaign.
According to Jeff Shaara in his book Jeff Shaara's Civil War Battlefields, the actual "High Water Mark" may not be the one that is commonly associated with the term:
I respect those who care deeply about paying homage to such noteworthy historical landmarks as the high-water mark. I speculate, however, that the copse of trees does not indicate the farthest advance of the Confederate troops that day. Drive just north, to the Bryan House. Walk to the stone wall on the left, peer over, and you will see the newest monument on the battlefield. This marks the spot where the battle flag of the 11th Mississippi was found as it lay across the stone wall. The 11th was part of the brigade commanded by Joe Davis, nephew of the Confederate president. By all information, a total of fourteen Mississippians reached this spot, farther into the Union position than the North Carolinians, at what is today labeled the high-water mark. The Mississippi regiment had four color bearers shot down, but the fifth made it, where he and the other survivors were captured. In a note of irony, this property was farmed by a freedman, Mr. Abraham Bryan. Thus did Davis's men see their assault end on the property of a freed slave.