By 11:30 a.m., the battlefield was temporarily quiet. On the Confederate side, Henry Heth faced an embarrassing situation. He had been under orders from General Lee to avoid a general engagement until the full Army of Northern Virginia had concentrated in the area. But his excursion to Gettysburg, ostensibly to find shoes, was essentially a reconnaissance in force conducted by a full infantry division. This indeed had started a general engagement and Heth was on the losing side so far. By 12:30 p.m., his remaining two brigades, under Brig. Gen. J. Johnston Pettigrew and Col. John M. Brockenbrough, had arrived on the scene, as had the division (four brigades) of Maj. Gen. Dorsey Pender, also from Hill's Corps.

Considerably more Confederate forces were on the way, however. Two divisions of the Second Corps, commanded by Lt. Gen. Richard S. Ewell, were approaching Gettysburg from the north, from the towns of Carlisle and York. The five brigades of Maj. Gen. Robert E. Rodes marched down the Carlisle Road but left it before reaching town to advance down the wooded crest of Oak Ridge, where they could link up with the left flank of Hill's Corps. The four brigades under Maj. Gen. Jubal A. Early approached on the Harrisburg Road. Union cavalry outposts north of the town detected both movements. Ewell's remaining division (Maj. Gen. Edward "Allegheny" Johnson) did not arrive until late in the day.^[19]

On the Union side, Doubleday reorganized his lines as more units of the I Corps arrived. First on hand was the Corps Artillery under Col. Charles S. Wainwright, followed by two brigades from Doubleday's division, now commanded by Brig. Gen. Thomas A. Rowley, which Doubleday placed on either end of his line. The XI Corps arrived from the south before noon, moving up the Taneytown and Emmitsburg Roads. Maj. Gen. Oliver O. Howard was surveying the area from the roof of the Fahnestock Brothers' dry-goods store downtown at about 11:30^[20] when he heard that Reynolds had been killed and that he was now in command of all Union forces on the field. He recalled: "My heart was heavy and the situation was grave indeed, but surely I did not hesitate a moment. God helping us, we will stay here till the Army comes. I assumed the command of the field."^[21]

Howard immediately sent messengers to summon reinforcements from the III Corps (Maj. Gen. Daniel E. Sickles) and the XII Corps (Maj. Gen. Henry W. Slocum). Howard's first XI Corps division to arrive, under Maj. Gen. Carl Schurz, was sent north to take a position on Oak Ridge and link up with the right of the I Corps. (The division was commanded temporarily by Brig. Gen. Alexander Schimmelfennig while Schurz filled in for Howard as XI Corps commander.) The division of Brig. Gen. Francis C. Barlow was placed on Schurz's right to support him. The third division to arrive, under Brig. Gen. Adolph von Steinwehr, was placed on Cemetery Hill along with two batteries of artillery to hold the hill as a rallying point if the Union troops could not hold their positions; this placement on the hill corresponded with orders sent earlier in the day to Howard by Reynolds just before he was killed.^[22]

However, Rodes beat Schurz to Oak Hill, so the XI Corps division was forced to take up positions in the broad plain north of the town, below and to the east of Oak Hill.^[23] They linked up with the I Corps reserve division of Brig. Gen. John C. Robinson, whose two brigades had been sent forward by Doubleday when he heard about Ewell's arrival.^[24] Howard's defensive line was not a particularly strong one in the north.^[25] He was soon outnumbered (his XI Corps, still suffering the effects of their defeat at the Battle of Chancellorsville, had only 8,700 effectives), and the terrain his men occupied in the north was poorly selected for defense. He held out some hope that reinforcements from Slocum's XII Corps would arrive up the Baltimore Pike in time to make a difference.^[26]