Land warfare changed from the static front lines of trench warfare of World War I, which had relied on improved artillery that outmatched the speed of both infantry and cavalry, to increased mobility and combined arms. The tank, which had been used predominantly for infantry support in the First World War, had evolved into the primary weapon.[404] In the late 1930s, tank design was considerably more advanced than it had been during World War I,[405] and advances continued throughout the war with increases in speed, armour and firepower.[citation needed] At the start of the war, most commanders thought enemy tanks should be met by tanks with superior specifications.[406] This idea was challenged by the poor performance of the relatively light early tank guns against armour, and German doctrine of avoiding tank-versus-tank combat. This, along with Germany's use of combined arms, were among the key elements of their highly successful blitzkrieg tactics across Poland and France.[404] Many means of destroying tanks, including indirect artillery, anti-tank guns (both towed and self-propelled), mines, short-ranged infantry antitank weapons, and other tanks were used.[406] Even with large-scale mechanisation, infantry remained the backbone of all forces,[407] and throughout the war, most infantry were equipped similarly to World War I.[408] The portable machine gun spread, a notable example being the German MG34, and various submachine guns which were suited to close combat in urban and jungle settings.[408] The assault rifle, a late war development incorporating many features of the rifle and submachine gun, became the standard postwar infantry weapon for most armed forces.[409]