Hood's division consisted of four brigades (BG Jerome B. Robertson supported by BG George T. Anderson on the left, with BG Evander Law supported by BG Henry L. Benning on the right). Law's 15th Alabama and the 47th Alabama Regiments were charged with clearing Big Round Top of Union troops. Earlier, Sickles had positioned the Union 2nd U.S. Sharpshooters at the foot of Big Round Top and this same unit contested the Confederate approach. After climbing and hard fighting, Colonel William C. Oates' 15th Alabama Regiment forced the 2nd U.S. to retreat up and over the hill. After finally reachin the top, Oates surveyed his position and let his men rest for a few minutes. Oates' position was 305 feet above the battlefield and provided excellent coverage of the immediate area. Oates noticed very few Union troops on Little Round Top except a small number of Union signalmen who were observing Confederate troops movements.

As his men rested, Captain L.R. Terrell of Law's staff arrived (to Oates' surprise, he had somehow managed to climb the hill on horseback). Terrell brought news that General Hood had been wounded and BG Law was now in command. Law's orders were to immediately assault Little Round Top and secure a foothold in the Union's southern flank. Oates moved to occupy Little Round Top and encountered no resistance at all and in fact was joined by Law's 4th Alabama and Robertson's 4th and 5th Texas Regiments. To Law's left, Benning and Anderson attacked Devil's Den. But, just as Oates began to climb Little Round Top's southern slope, it came under what Oates recalled as "the most destructive fire I ever saw."

Law's Brigade came upon Colonel Strong Vincent's and Weed's Brigades who had arrived just 10 minutes earlier. Oates' Regiment had ran up against Colonel Joshua Chamberlain's 20th Maine . (These two regiments are focused because they are the absolute anchor of both army's flank - to turn Chamberlain's Regiment would be to lose Little Round Top and possibly the Union's left flank). Chief engineer Gouverneur Warren (who had been sent by Meade to survey the area) ordered Captain James Smith's 4th NY Battery above the Devil's Den to fire a shell into the area just north of Big Round Top. As Warren stated, "The motion revealed to me the glistening of gun barrels and bayonets of the enemy's line of battle, already formed and far outflanking the position of any of our troops." Warren now realized that the key lie with Little Round Top and immediately requested troops to be positioned on the hill. Sickles, at this time, was being attacked by McLaws and could not spare any extra units, but Meade had ordered Sykes' V Corps to reinforce Sickles' position. Lt. Ranald S. Mackenzie (an aid of Warren) finally found Sykes and notified him of the impending catastrophe. Sykes ordered BG James Barnes' Division to occupy Little Round Top. At the front of Barnes' Division was Colonel Vincent's Brigade followed by Hazlett's Brigade. Chamberlain's 20th Maine of Vincent's Brigade would be awarded the Medal of Honor for his following actions.

Oates' Regiment ascended Little Round Top at nearly the same time that Chamberlain's Regiment began positioning itself. When Chamberlain's Regiment first observed Oates, the 20th Maine opened fire and sent the 15th Alabama scurrying for cover. Then, Oates regrouped and ordered a rush up the hill towards Chamberlain's position. Oates managed to threaten to Chamberlain's left flank, so the 20th Maine left flank was ordered to withdraw so that the Regiment formed a "V". Over and over Oates rushed Chamberlain's position and threatened to overwhelm the Regiment. But, as the 20th Maine's men were expending their last ammunition, Chamberlain ordered to fix bayonets and prepare to charge. Before the Confederates were able to assess the situation, Chamberlain's Regiment came charging down Little Round Top and overwhelmed Oates' Regiment. Oates, unable to maintain his position, ordered a retreat off the hill. BG Stephen H. Weed's Brigade soon reinforced the Union position on Little Round Top and forced the Confederate attackers off the hill.

The Confederates were now able to attack Little Round Top from the southwest, but Vincent's men (reinforced by Stephen H. Weed's Brigade) were able to secure the hill. The Union had suffered 780 casualties (over 34%).

THE DEVIL'S DEN

The Devil's Den was an enormous collection of boulders and rocks that were occupied by Captain James E. Smith's 4th NY Battery and BG Hobart Ward's Brigade. Hood attacked Devil's Den with Benning's and Anderson's Brigades. The 1st Texas and 15th Georgia charged Smith's Battery and nearly overwhelmed it when the 124th NY under command of Major James Cromwell led a counterattack. Cromwell was instantly killed and relieved by Col. Augustus Van Horne Ellis who was also killed. The fighting in the Devil's Den was confused and desperate as both sides struggled to occupy the mass of boulders. Eventually, the Confederates managed to capture three of Smith's guns and occupy the Den and the ridge above.