William Swinton, Campaigns of the Army of the Potomac

It has been seen that, at the same time Hood thrust his right through the interval between Sickles' left flank and Round Top, and entered upon the contest for the possession of that point, he also assailed the portion of the Third Corps, line held by Birney's division. In this attack he was joined by Longstreet's other division under General McLaws, so that this effort was directed against the entire left and centre of the Third Corps, from its left flank near Round Top, forward to the salient in the peach orchard on the Emmettsburg road. But it happened that Longstreet's line as formed did not cover the entire front of Sickles' corps (for Hood's point of attack was quite to the Confederate right, and Longstreet had only McLaws' division in addition), and it failed to cover it by about the front held by the right division under General Humphreys. Where Longstreet's line terminated, however, the prolongation towards the Union right was continued by Hill's corps, so that Humphreys had part of that corps in his front. But Hill's duty was, while Longstreet attacked, to make demonstrations and only assault in case of a good opportunity. Thus it came about, that, when Longstreet, after the development of Hood's attack, advanced McLaws' division on the left of Hood, the brunt of the assault fell upon Sickles' centre and left under Birney; Humphreys' division being for the time unassailed.

The onset of Hood and McLaws upon Birney's front was made with great vigor, compelling General Sickles immediately to call for re-enforcements; and it was in response to this request that General Barnes' division of the Fifth Corps had been thrown out in support at the time General Warren detached from this division the brigade of Vincent to hold Little Round Top. Its other two brigades, under Colonels Tilton and Sweitzer, hastened to the support of Birney's hard-pressed troops on the advanced line; and General Humphreys, who held the right of the Third Corps, but had not yet been attacked, sent one of his own brigades under Colonel Burling to still further help.

The heaviest pressure of the hostile attack fell upon that