Law led his brigade through the Peninsula Campaign and the Seven Days Battles. At Gaines' Mill, he and fellow brigade commander Brig. Gen. John Bell Hood achieved fame by breaking the center of the Union line. They attacked in tandem again at the Battle of Malvern Hill four days later, but were defeated decisively. In the Northern Virginia Campaign, at the Second Battle of Bull Run, Law and Hood were used again as the primary assaulting force in Longstreet's surprise attack against the Union left flank, almost destroying Maj. Gen. John Pope's Army of Virginia.
In the Maryland Campaign, at the Battle of Antietam, Law's Brigade defended against the Union attack through the Cornfield at high cost—454 killed and wounded. Law was promoted to brigadier general on October 3, 1862. At the Battle of Fredericksburg in December, he saw little action.
On July 3, Law's men were at the extreme right of the Confederate line and defended against a suicidal cavalry attack made by Union troops of Brig. Gen. Judson Kilpatrick's division, led by their brigade commander Brig. Gen. Elon J. Farnsworth.
Gen. Longstreet disapproved of the contents of Law's official report on the battle and refused to accept it, and no copies seem to have survived.. Years later, Law published his own account of the fighting on July 2, "The Struggle For 'Round Top'", in Battles and Leaders of the Civil War.
Despite Longstreet's praise for Law's performances in previous battles, the two became involved in bitter disputes, some of which involved professional jealousy between Law and Brig. Gen. Micah Jenkins, a favorite of Longstreet's. At different times and places, Longstreet had promised both Law and Jenkins command of Hood's division, should that command billet ever open. Law had served in Hood's division since its organization, and had commanded it successfully at Gettysburg and Chickamauga. Jenkins was new to the division and had never commanded it, but his commission as a brigadier general pre-dated Law's, and when Jenkins' brigade was attached to Hood's division in September 1863, shortly after Chickamauga, with Hood absent due to wounds, Law had to turn command of Hood's division over to Jenkins.
Hood's division accompanied Bragg's army to the siege of Chattanooga. By late October 1863, Law's brigade was detached from Hood's division and the army, guarding Brown's Ferry over the Tennessee River in what is known as Lookout Valley. While Law was on leave, visiting the wounded Hood, division commander Jenkins stripped the defenses at Brown Ferry of over half the units, despite intelligence of enemy activity and pleas for reinforcements from Col. William Oates, commanding the 15th Alabama Regiment, one of the two regiments still posted near Brown's Ferry. On October 24, 1863, Union troops forced a passage of Brown's Ferry and overwhelmed its defenders. A few days later, Federal reinforcements—the XI and XII Corps from the Army of the Potomac—arrived at the other end of Lookout Valley, at Wauhatchie Station. The arrival of these fresh Federal troops, combined with the Federal possession of Brown's Ferry, enabled U.S. Lt. Gen. Ulysses S. Grant to open his "cracker line" and feed his starving troops in Chattanooga.
Confederate Gen. Bragg recognized that the "cracker line" would spell the end of the siege of Chattanooga, and on October 28, ordered Longstreet to take his corps and seize control of Lookout Valley. Longstreet decided to send only Hood's division to deal with the two enemy corps. Gen. Jenkins quickly planned a night attack on the railhead at Wauhatchie, to be made by two brigades, his own under Col. John Bratton, and Brig. Gen. Henry L. Benning's. Simultaneous to the attack at Wauhatchie, and over a mile distant, a holding action near Brown's Ferry was to be made by the Texas brigade and Law's brigade. Already outnumbered, Jenkins further aggravated his situation by failing to utilize Brig. Gen. George T. Anderson's brigade, also of Hood's division, and a sizable portion of the Hampton Legion Infantry, of his own brigade. As the battle broke out, division commander Jenkins rode to Wauhatchie, on the extreme left of his widely dispersed division, instead of placing himself in a position where he could attempt to coordinate all of his troops. The ensuing engagement was a Confederate defeat. Jenkins later claimed that Law quit his holding mission prematurely; Law, and Brig. Gen. Robertson, commanding the Texas brigade, claimed they acted in accordance with orders. This controversy brought tensions between Jenkins and Law to the boiling point, and has never been settled.
Jenkins continued in command of Hood's division, through Longstreet's East Tennessee campaign of November–December 1863. Hood's division continued to malfunction under Jenkins, who again blamed Law for the difficulties, particularly at Campbell's Station. The command situation in Hood's division, and Longstreet's Corps, deteriorated markedly through March 1864, with Law, Maj. Gen. Lafayette McLaws, and at least one other brigadier general arrested and court martialed by Longstreet; Longstreet's charges against his subordinates were not sustained by the Confederate War Department.
The continued stress resulted in Law's request for resignation, which he offered to deliver to Richmond in person. While there, he visited Hood, who talked Law out of resigning and used his influence to keep the War Department from accepting it. On Law's return to his brigade, still in East Tennessee, Longstreet ordered Law's arrest for insubordination. The men of Law's brigade had by this time had enough, and all but one of the colonels requested their regiments' transfer, with the whole brigade, to Alabama. Longstreet attempted to retaliate by leaving them in Tennessee when the rest of his corps rejoined the Army of Northern Virginia. General Robert E. Lee, however, ordered Law and the Alabamians back to his army. Hood had been promoted, and a new commander, Charles W. Field, was assigned to command Hood's old division, after which the division made a remarkable turn around, regaining in a month the efficiency it had last shown at Chickamauga.
While his brigade fought in the Siege of Petersburg, Law was transferred to brigade command in Maj. Gen. Wade Hampton's Cavalry Corps, stationed in South Carolina, where he finished the war. When Maj. Gen. Matthew Butler was wounded at the Battle of Bentonville, Law exercised command of Butler's division until its commander returned to active duty.