xix army corps saw initial action on the first day of the invasion, 1 september 1939. although the polish military had been aware of the german troop concentrations, the germans were nevertheless able to seize the initiative with a surprise attack. polish mobilization was not yet completed. while the germans had a considerable manpower advantage as well (~1.5 million versus ~1 million), the main german military advantage were the numerical superiority in terms of equipment, including armored vehicles, field guns and military aircraft. precise numbers are difficult to come by for the only partially mobilized and bureaucratically ill-prepared polish defenders.