When was the first electrical intra-battlefield communication?
Broadly, the United States Civil War.. Definitely Sept 17, 1862, arguable July 21, 1861.
July 21, 1861 Battle of First Manassas Confederate Generals Beauregard and Johnston coordinated the convergence of their two armies to meet a Union offensive at the first Battle of Manassas. Johnson would further innovate by being the first commander to use Railroads to transport his army into battle. Now that could be argued as occurring before the battle of First Manassas, but I would say it was after Beauregard had met the union forces and was reacting to them. So I would say it counts.
Alternatively, The Union's General McClellan would use the telegraph at the battle of Anietam Sept 17, 1862 to coordinate resupply of artillery shells and ammunition during that action. Directly communicating between units during the action at Anietam.
General Beauregard in Command of the Confederate's Army of the Potomac, used the telegraph to inform General Johnston in command of The Army of the Shenandoah, on his troops falling back on the stream of Bull Run. General Johnston would move his army to support Beauregard at the Battle of First Manassas one of the first major battles of the Civil War. The battle of Manassas
was fought over an important railroad junction. And since telegraphs lines in those days were placed next to railroad tracks. The Battle of Manassas had a telegraph line present on the battlefield, for General Beauregard to use. General Johnston was stationed with his army at Harpers Ferry at the other end of the Winchester and Potomac Railroad
First Battle of Manassas
Indeed, on July 17, in the face of the Union advance, Beauregard informed Confederate president Jefferson Davis that "the enemy have assailed my outposts in heavy force" and that he had "fallen back on the line of Bull Run." Word went out by telegraph to Johnston that the anticipated Union advance had begun. Johnston, confident that Patterson would stay put, immediately agreed to shift his forces to Manassas.
The Union's General McClellan used the Telegraph during the Battle of Anietam September 1862 in Maryland to coordinate resupply of his troops with ammunition and artillery shells during that action. Anietam was special for the purposes of this question. Telegraphs of coarse are laborious things to install on a battlefield, and can only be of use if you know where the battle is going to be and you have a commander obsessed with leaving nothing to chance, wanting to control every minute detail of the battle. At Anietam you have both. General McClellan had Lee's attack plans before the battle. He knew where, when, and how Lee was going to attack. General McClellan was also famous for obsessively planning every aspect of his battles. Some would say he over thought them.
The Telegraph -Essential Civil War Curriculum
“Hardly a day intervened when General Grant did not know the exact state of facts with me, more than fifteen hundred miles off, as the wires ran.” McClellan adroitly used the telegraph to resupply his troops with bullets and shells in the midst of the Battle of Antietam, Maryland, in September 1862.
During the battle of Spotsylvania in the Wilderness Campaign of May 1864, Major General George Gordon Meade used the telegraph to reinforce Major General Winfield Scott Hancock’s II Corps after it had come under heavy Confederate counterattack. Stanton relied on the military telegraph to monitor the actions of generals in the field, and Lincoln spent countless hours in the War Department telegraph office adjoining Stanton’s office. For the first time in the history of warfare, the telegraph helped field commanders to direct real-time battlefield operations and permitted senior military officials to coordinate strategy across large distances. These capabilities were key factors in the North's victory.