The Battle of Kennesaw Mountain was fought on June 27, 1864, during the Atlanta Campaign of the American Civil War. The most significant frontal assault launched by Union Major General William T. Sherman against the Confederate Army of Tennessee under General Joseph E. Johnston, it produced a tactical defeat for the Union forces but failed to deliver the result that the Confederacy desperately needed: a halt to Sherman's advance on Atlanta, Georgia.

Sherman's 1864 campaign against Atlanta began with a series of flanking maneuvers that compelled Johnston's forces to withdraw from heavily fortified positions with minimal casualties on either side. After two months and 70 miles (110 km) of such maneuvering, Sherman's path was blocked by imposing fortifications on Kennesaw Mountain, near Marietta, Georgia. The Union general chose to change his tactics and ordered a large-scale frontal assault on June 27. Major General James B. McPherson feinted against the northern end of Kennesaw Mountain, while his corps under Major General John A. Logan assaulted Pigeon Hill on its southwest corner. At the same time, Major General George H. Thomas launched strong attacks against Cheatham Hill at the center of the Confederate line. Both attacks were repulsed with heavy losses, but a demonstration by Major General John M. Schofield achieved a strategic success by threatening the Confederate army's left flank, prompting yet another Confederate withdrawal toward Atlanta and the removal of General Johnston from command of the army.

In March 1864, Ulysses S. Grant was promoted to lieutenant general and named general in chief of the Union Army. He devised a strategy of multiple, simultaneous offensives against the Confederacy, hoping to prevent any of the rebel armies from reinforcing the others over interior lines. The two most significant of these were by Major General George G. Meade's Army of the Potomac, accompanied by Grant himself, which would attack Robert E. Lee's army directly and advance toward the Confederate capital of Richmond, Virginia; and Major General William T. Sherman, replacing Grant in his role as commander of the Military Division of the Mississippi, who would advance from Chattanooga, Tennessee, to Atlanta.

Both Grant and Sherman initially had objectives to engage with and destroy the two principal armies of the Confederacy, relegating the capture of important enemy cities to a secondary, supporting role. This was a strategy that President Abraham Lincoln had emphasized throughout the war, but Grant was the first general who actively cooperated with it. As their campaigns progressed, however, the political importance of the cities of Richmond and Atlanta began to dominate their strategy. By 1864, Atlanta was a critical target. The city of 20,000 was founded at the intersection of four important railroad lines that supplied the Confederacy and was a military manufacturing arsenal in its own right. Atlanta's nickname of "Gate City of the South" was apt—its capture would open virtually the entire Deep South to Union conquest. Grant's orders to Sherman were to "move against Johnston's Army, to break it up and to get into the interior of the enemy's country as far as you can, inflicting all the damage you can against their war resources."

Sherman's force of about 100,000 men was composed of three subordinate armies: the Army of the Tennessee (Grant's and later Sherman's army of 1862–63) under Major General James B. McPherson; the Army of the Cumberland under Major General George H. Thomas; and the relatively small Army of the Ohio (composed of only the XXIII Corps) under Major General John M. Schofield. Their principal opponent was the Confederate Army of Tennessee, commanded by General Joseph E. Johnston, who had replaced the unpopular Braxton Bragg after his defeat in Chattanooga in November 1863. The 50,000-man army consisted of the infantry corps of lieutenant generals William J. Hardee, John Bell Hood, and Leonidas Polk, and a cavalry corps under Major General Joseph Wheeler.

Sherman's campaign began on May 7, as his three armies departed from the vicinity of Chattanooga. He launched demonstration attacks against Johnston's position on the long, high mountain named Rocky Face Ridge while McPherson's Army of the Tennessee advanced stealthily around Johnston's left flank toward the town of Resaca, and Johnston's supply line on the Western & Atlantic Railroad. Unfortunately for Sherman, McPherson encountered a small Confederate force entrenched in the outskirts of Resaca and cautiously pulled back to Snake Creek Gap, squandering the opportunity to trap the Confederate army. As Sherman swung his entire army in the direction of Resaca, Johnston retired to take up positions there. Full scale fighting erupted in the Battle of Resaca on May 14–15 but there was no conclusive result and Sherman flanked Johnston for a second time by crossing the Oostanaula River. As Johnston withdrew again, skirmishing erupted at Adairsville on May 17 and more general fighting on Johnston's Cassville line May 18–19. Johnston planned to defeat part of Sherman's force as it approached on multiple routes, but Hood became uncharacteristically cautious and feared encirclement, failing to attack as ordered. Encouraged by Hood and Polk, Johnston ordered another withdrawal, this time across the Etowah River.

Johnston's army took up defensive positions at Allatoona Pass south of Cartersville, but Sherman once again turned Johnston's left as he temporarily abandoned his railroad supply line and advanced on Dallas. Johnston was forced to move from his strong position and meet Sherman's army in the open. Fierce but inconclusive fighting occurred on May 25 at New Hope Church, May 27 at Pickett's Mill, and May 28 at Dallas. By June 1, heavy rains turned the roads to quagmires and Sherman was forced to return to the railroad to supply his men. Johnston's new line (called the Brushy Mountain Line) was established by June 4 northwest of Marietta, along Lost Mountain, Pine Mountain, and Brush Mountain. On June 14, following eleven days of steady rain, Sherman was ready to move again. While on a personal reconnaissance, he spotted a group of Confederate officers on Pine Mountain and ordered one of his artillery batteries to open fire. Lieutenant General Leonidas Polk, the "Fighting Bishop", was killed and Johnston withdrew his men from Pine Mountain, establishing a new line in an arc-shaped defensive position from Kennesaw Mountain to Little Kennesaw Mountain. Hood's corps attempted an unsuccessful attack at Peter Kolb's farm (the Battle of Kolb's Farm) south of Little Kennesaw Mountain on June 22. Major General William W. Loring succeeded in temporarily commanding Polk's corps.

Sherman was in a difficult position, stalled 15 miles (24 km) north of Atlanta. He could not continue his strategy of moving around Johnston's flank because of the impassable roads, and his railroad supply line was dominated by Johnston's position on the top of 691 feet (211 m) Kennesaw Mountain. He reported to Washington "The whole country is one vast fort, and Johnston must have at least 50 miles (80 km) of connected trenches with abatis and finished batteries. We gain ground daily, fighting all the time. ... Our lines are now in close contact and the fighting incessant, with a good deal of artillery. As fast as we gain one position the enemy has another all ready. ... Kennesaw ... is the key to the whole country." Sherman decided to break the stalemate by attacking Johnston's position on Kennesaw Mountain. He issued orders on June 24 for an 8:00 a.m. attack on June 27.