The infantry remained foot soldiers throughout the war; artillery also remained primarily horse-drawn. The motorized formations received much attention in the world press in the opening years of the war, and were cited as the main reason for the success of the German invasions of Poland (September 1939), Denmark and Norway (1940), Belgium, France and Netherlands (May 1940), Yugoslavia (April 1941) and the initial stages of Operation Barbarossa, the invasion of the Soviet Union (June 1941). However, their motorized and tank formations accounted for only 20% of the Heer's capacity at their peak strength.The army's lack of trucks (and of petroleum to run them) severely limited infantry movement, especially during and after the Normandy invasion when Allied air-power devastated the French rail network north of the Loire. "Panzer" movements also depended on the rail, since driving a tank long distances wore out its tracks.