France and Britain declared war on Germany on 3 September, but failed to provide any meaningful support. The German-French border saw only a few minor skirmishes, and most German forces, including 85% of armoured forces, were engaged in Poland. Despite some Polish successes in minor border battles, the German technical, operational and numerical superiority forced the Polish armies to retreat from the borders towards Warsaw and Lwów. The "Luftwaffe" gained air superiority early in the campaign. By destroying communications, the "Luftwaffe" increased the pace of the advance which overran Polish airstrips and early warning sites, causing logistical problems for the Poles. Many Polish Air Force units ran low on supplies, and 98 of their number withdrew into neutral Romania. The Polish initial strength of 400 was reduced to 54 by 14 September and air opposition virtually ceased, with the main Polish air bases destroyed during the first 48 hours of the war.