Having caught the Federals off guard, Gen. Robert E. Lee determined to attack the Army of the Potomac on July 2. Gen. James Longstreet's newly arrived corps of infantry was march south and attack the Union's exposed left flank on Cemetery Ridge. Longstreet, who disagreed with Lee's plan, did not get his troops into position until mid-afternoon. As Longstreet's attack column was getting in place, Union Third corps chief, Gen. Daniel Sickles advanced his troops off of Cemetery Ridge to take advantage of what he deemed to be ground better suited for defense.
Around 4:30 Gen. John B. Hood's Confederate division launched its much-anticipated assault. Spearheaded by Texans, Arkansans, and Alabamians, Hood's men swept over Sickles' men occupying the boulder-strewn Devil's Den and made their way toward a rocky hill known as Little Round Top. Seeing this movement, the Federals' chief engineer Gen. Gouverneur K. Warren worked feverishly to rush troops to the imperiled sector. Two brigades of the Fifth Corps, Col. Strong Vincent and Col. Patrick O'Rorke arrived in the nick of time and were able to halt the Rebel advance, and secure the Union left.
While the Confederates assaulted the Union center, Federal cavalry commander Gen. Alfred Pleasonton issued an ambiguous directive to his subordinate on the southern part of the field, Gen. Hugh Judson Kilpatrick. Kilpatrick, with an order to strike the enemy "at the earliest opportunity," launched a series of piecemeal mounted attacks against the Confederates astride the Emmitsburg Road and at the foot of Big Round Top.