The Germans developed a plan to rapidly move their forces to surround the Second Army as it moved northward over some particularly hilly terrain. The danger was that the First Army would turn to their aid, thereby flanking the German forces. However, the Russians broadcast their daily marching orders "in the clear" on the radio, and the Germans learned that the First Army was continuing to move away from the Second. Using railways in the area, the German forces maneuvered and eventually surrounded and destroyed the Second Army at the Battle of Tannenberg between 26 and 30 August 1914.