But in the opening offensives of the Second World War, the German success of armored forces concentrated and used in a fast-moving offensive shocked military observers. Even to armies which had previously experimented with large-scale mechanized warfare, the effects were remarkable; the collapse of Poland in 1939, followed by the defeat of the French Army and the British Expeditionary Force in France in 1940, gave rise to an impression that massed tank forces were effectively invincible when used against unprepared defenders.