Complex sentence: When asked for the time between when he issued an order and when it was put into effect in the field, his estimate was 48 hours--this at a time when motorized German forces were able to overrun the Netherlands in five days.
Split into simpler sentences: When asked for the time between when he issued an order and when it was put into effect in the field, his estimate was 48 hours. This at a time when motorized German forces were able to overrun the Netherlands in five days.