There were clashes at Pine Mountain on 11–14 June and Lost Mountain on 15–17 June. In the Battle of Kennesaw Mountain on 27 June, Newton's division made the main IV Corps effort. The brigades of Wagner and Harker led the assault with Kimball's brigade in close support. Confederate fire was intense and the assault failed with heavy losses. Lieutenant Colonel Chandler was killed in the attack and Major Smith assumed command of the 88th Illinois. During the Battle of Peachtree Creek on 20 July, the Confederates launched a major assault which first struck Newton's division and was repulsed. The regiment participated in the siege of Atlanta from 22 July to 25 August, the flank movement on 25–30 August, and the Battle of Jonesborough on 31 August – 1 September.