which won the Luftwaffe air-superiority. By destroying communications the Luftwaffe increased the pace of the advance which overran Polish airstrips and early warning sites and caused logistical problems for the Polish forces. Many Polish Air Force units were by then low on supplies, 98 of their number withdrew into neutral (at that time) Romania. Their initial strength of 397 had been reduced to just 54 by 14 September and air opposition virtually ceased. Part of the Luftwaffe's operations involved the destruction of the small but modern Polish Navy. The Luftwaffe had few units capable of effective anti-shipping operations. One of these units was 4.(St)/TrGr 186 – a Stuka unit which originally had been trained to operate from the German aircraft carrier Graf Zeppelin. 4.(St)/TrGr 186's most notable success was to sink the destroyer Wicher and mine-layer Gryf. The Luftwaffe had been successful in neutralizing Polish air and sea power within first days of campaign, and with air superiority established the Luftwaffe was free to concentrate on attacking supply and communication links with devastating effect. The outdated Polish Army was denied what mobility it had left as traveling by road became perilous and the rail networks had been largely destroyed. The Stuka became the symbol of the German war machine throughout the campaign. Operating without opposition, it supported the Panzer divisions by acting as flying artillery. On one occasion six Polish divisions trapped by encircling German forces were forced to surrender after a relentless four-day assault by StG 51, StG 76 and StG 77. Employed in this assault was the 50 kg fragmentation bombs which caused appalling damage to enemy ground troops. Demoralized, the Poles surrendered. The Wehrmacht however was given a shock soon afterwards. The Polish 'Poznan' and 'Pomorze' armies counter-attacked, threatening to break the flank of the German 8. Armee, and to cut off the 10.Armee. The Luftwaffe was called upon for a maximum effort in what became known as the Battle of Bzura. The Luftwaffe's offensive broke what remained of Polish resistance in an "awesome demonstration of air power". The Luftwaffe quickly destroyed the bridges across the Bzura river. Afterward the Polish forces were trapped out in the open, and were attacked by wave after wave of Stukas, dropping 50 kg 'light bombs' which caused huge numbers of casualties. The Polish flak positions ran out of ammunition and they retreated to the forests but were then 'smoked out'