along the causeway leading from the bridge in Wizna. Before September 1, 1939, only 16 bunkers were built out of 60 planned. Six bunkers were made of heavy concrete with reinforced steel cupolas weighing 8 tons each, armed with machine guns and anti-tank artillery. Two were light concrete bunkers, armed with machine guns only. The remaining eight were ad hoc machine gun pillboxes, protected mostly by sandbags and earthworks. Four additional heavy bunkers were under construction when World War II started. In addition, the area was reinforced with trenches, anti-tank and anti-personnel obstacles, barbed wire lines and land mines. There were also plans to break the dams on the Biebrza and Narew rivers to flood the area, but the summer of 1939 was one of the driest seasons in Polish history and the water level was too low. Although not all bunkers were ready by the beginning of the war, the Polish lines of defence were well-prepared. The walls of an average bunker, 1.5 metres thick and reinforced with 20-centimetre-thick steel plates, could withstand a direct hit from even the heaviest guns available to the Wehrmacht at the time. The bunkers were situated on hills which gave good visibility of all the advancing forces. Opposing forces The Polish defensive line was initially manned by a single battalion from the 71st Infantry Regiment, commanded by Major Jakub Fober. However, shortly before the outbreak of World War II it was reinforced with a machine gun company from Osowiec Fortress under Captain Władysław Raginis, as well as numerous smaller detachments from a variety of units. On September 2, 1939, the III/71 battalion departed for Osowiec, and Fober passed command over Wizna to Raginis and his men. Altogether, the Polish defensive position was manned by 720 men: 20 officers and 700 NCOs and privates. However, some sources claim that the Polish unit was even weaker, no more than 360 men strong. Although the Polish units were almost entirely composed of conscripts mobilised in August 1939 rather than professional soldiers, their morale was very high. After the war, Guderian had trouble explaining why his corps was stopped by such a small force. In his memoirs he attributes the delay to his officers "having trouble building bridges across the rivers". During the Nuremberg Trials he remarked that Wizna was "well-defended by a local officer school". Before the battle On September 1, 1939, the Polish Defensive War