In their Invasion of Poland during September 1939, German forces applied a narrow cooperation between large armoured units — of the "Panzerwaffe" and the Cavalry — and "active" infantry divisions to break the Polish defensive lines and pursue the defeated enemy forces. The more limited and dispersed Polish armoured units were quickly destroyed. The Red Army, invading the east of Poland, also deployed armoured divisions. At the time, the swift collapse of the Polish army was seen as the result of an armoured "Blitzkrieg". However, later it has been argued that the campaign was largely an instance of the classical nineteenth century German concept of the "Annihilation Battle", in which the role of deep strategic armoured penetrations was limited.