This totally unexpected setback led to panic in the German military leadership, which ordered the undefended city center of Rotterdam to be wiped out (see below) in order to force an off-the-battlefield solution and stop the effective resistance by the Dutch forces. Before this terror bombardment, the Royal Netherlands Navy managed to ship some 1300 captured German crack troops to England, providing their allies with first-hand intelligence about this novel type of airborne warfare. the Grebbe line, a north-south line some 50 kilometres (31 miles) east of the capital Amsterdam, from Amersfoort to the Waal, fortified, with field guns, and extensive inundations; the Dutch only surrendered after three days of hard and aggressive fighting, known as the Battle of the Grebbeberg, with heavy losses on both sides. Having taken the Grebbeberg, the German forces were confronted by the next setback: during the battle, the old Hollandic Water Line, which was designed to make any incursion into Fortress Holland impossible had been inundated and thus reactivated.