The city of Liege occupies a strategic position at the confluence of the Meuse and Ourthe rivers in eastern Belgium. It became the site of the first battle of the Great War. In accordance with the Schlieffen Plan, German soldiers avoided the heavily defended Franco-German border and stormed into neutral Belgium. Liege was defended by a series of 12 forts that were spaced around the city's outskirts. The Germans anticipated no difficulty dealing with the small Belgian army, but were amazed by the stubborn resistance encountered on the first day of battle. The Germans made little progress and suffered heavy casualties. Deciding to temporarily bypass the forts, the Germans turned their attention to the city itself. They staged the first air raid in history by dispatching a zeppelin whose crew dropped 13 bombs out of the windows upon the startled residents below. The city surrendered shortly thereafter. Eventually the protective ring of forts fell to the Germans, but only after they brought in newly developed howitzers (short barreled cannon) from the vaunted Krupp works. "Big Bertha" (one of several siege howitzers by that name) dispatched shells from its 16-inch barrel to reduce the Belgian positions. Liege and its environs were in German hands by August 16. The following day, the invaders moved on to Antwerp. Brussels fell without resistance on August 20. By the 23rd, all of Belgium was open to the German advance. Belgium paid dearly for choosing to defend its neutrality, but played a key role by slowing the German forces on the Western Front.