After the discovery on June 30 that Gettysburg was occupied by Brigadier General John Buford's division of Federal cavalry, the Confederates on July 1 sent the divisions of Major General Henry Heth and Major General William Pender of Hill's Corps, down the Chambersburg Road to drive Buford away and occupy Gettysburg.

The battle began at 5.30 a.m., when shots were exchanged over Marsh Creek. In the face of Buford's resistance, Heth pushed on cautiously until he reached a point about two miles west of Gettysburg. Here he deployed two brigades in line, and pressed ahead; it was nearly 10 a.m. Federal General John F. Reynolds, commanding I Corps, arrived on the field at this point, and determined to engage Herb. He ordered I Corps and Major General Oliver 0. Howard's XI Corps to march to Gettysburg. Soon after 10.30 a.m., I Corps arrived and engaged Heth along McPherson's Ridge.

By 11.30 a.m., Heth had been defeated and forced to withdraw to Herr Ridge. Early in the action, Reynolds was killed, and field command devolved upon Howard. A lull now settled over the field as both sides brought up reinforcements. The Federal I Corps deployed to defend the western approaches to Gettysburg, while XI Corps formed up north of the town. Buford's cavalry covered the flanks. Howard left one division in reserve on Cemetery Hill. His strategy was simple: delay the Confederates long enough to enable the rest of the Federal army to concentrate.

Lee arrived on the field after noon. He had initially hoped to avoid a general engagement since the strength of the enemy was unknown, and the terrain in the Gettysburg area unfamiliar. But, soon after noon, Rodes's division of Ewell's Corps arrived on Oak Hill and attacked the right of I Corps.

At 2 p.m. Heth's division joined the attack on I Corps. At 3 p.m., the battle spread north of the town when Jubal Early's division of Ewell's Corps attacked down the Harrisburg Road and crushed the flank of XI Corps. At about the same time, west of Gettysburg, Pender's division relieved Heth and assaulted I Corps' position along Seminary Ridge.

By 4 p.m., both Federal corps were in retreat through Gettysburg to Cemetery Hill. Federal losses numbered slightly over 9,000, including some 3,000 captured, compared with Confederate losses of about 6,500. The day's action had resulted in a Confederate victory, but Federal forces held onto the high ground south of Gettysburg, where their position was soon strengthened by reinforcements.