Intricate warning systems have though always been used such as scouting towers with fires to signal incoming threats - this could occur at the tactical as well as the strategic level.[3] Although visual communication flew at the speed of light, it relied on a direct line of sight between the sender and the receiver.At the end of the 19th century the disparate units across any field were instantaneously joined to their commanders by the invention and mass production of the radio.[5] During the Interwar period the German army invented Blitzkrieg in which air, armor, and infantry forces acted swiftly and precisely, with constant radio communication.Advances in long-range communications aided the commander on the battlefield, for then they could receive news of any outside force or factor that could impact the conduct of a battle.