The Battle of Resaca was the first time all three of Sherman's armies were on the same field simultaneously

Beginning on March 18th, 1864, Sherman took command of the Mississippi Military Division, which was made up of soldiers from three distinct districts. Three infantry corps made up of 72,938 officers and soldiers made up Maj. Gen. George H. Thomas' Army of the Cumberland, which was the largest. Men from the Army of the Potomac who had been moved to relieve Chattanooga in 1863 made up the Twentieth Corps. The Ohio Army of Maj. Gen. John M. Schofield had just one corps, the Twenty-third, with about 12,805 soldiers. Finally, the Army of Tennessee under Maj. Gen. James McPherson, which included two corpsmen who had fought at Shiloh and Vicksburg for Grant and Sherman, totaled 24,380 men. All three of these armies had previously operated separately from one another until 1864. Under Sherman's command, however, these three previously independent forces would now operate together with one objective: the capture of Atlanta.

Sherman ordered his other two forces to advance through Snake Creek Gap and occupy a position opposite the Confederate positions near Resaca on May 9, after McPherson's Army of Tennessee failed to sever the railroad. So, on May 14 and 15, when the Federal forces were engaged, all three of these armies were together for the first time on a single battlefield.