Communication on the battlefield has always been a major concern of armies. In the early days, leaders could shout commands to their troops. Even with a relatively small number of men and close quarters, this became almost impossible.
Some armies developed a system of flags that could be waved during a battle that would pass on the orders of the general to their men. This increased the distance over which the commands could be given. It did rely on the men being able to see the flags. As the size of the battlefields grew the less valuable this method became.
Eventually, music became the standard. Drums and trumpets translated commands down the line and to anyone within earshot. Much more effective than flags, but as the size of armies grew so did the size of battlefields. Battles were being fought over miles now and even relaying orders from the leaders to the men either took too long or were too easily misunderstood.
During the Civil War, the telegraph changed everything. President Lincoln could stand in the War Office in Washington and get real-time updates of a battle in Tennessee. Heck if he wanted (and occasionally he did) he could give orders to Generals commanding on the front lines. (They loooved that.)
Fast forward a hundred years and the advent and proliferation of radios like the one above battles could be fought by men on one side of an ocean commanding men on the other. Today we have satellites and near instantaneous communications with nearly any point on the globe. We’ve come a long way.