Leonardo was to say, in his introductory letter to Ludovico Sforza, Duke of Milan, that, " I can make armoured cars, safe and unassailable, which will enter the serried ranks of the enemy with their artillery, and there is no company of men at arms so great that they will break it. And behind these the infantry will be able to follow quite unharmed and without any opposition." The tank design is well-known. It could be powered either through the use of horse, or by men hand cranking. The cranks were attached to trundle wheels, which then attached onto the driving wheels. It was considered that the second method of powering the vehicle was preferable as horses might be difficult to keep calm within the small confines of what would be a noisy area. His turtle-shaped design had a major flaw in that the front and rear wheels were geared to turn in opposite directions. Suggestions have been made that this could have been a deliberate mistake by Leonardo as he was against war and peaceable by nature. Leonardo was certainly not the first to work on the idea of a tank; it was quite a common area of research in his day with several sail-powered tanks being designed. Still, it was to be a further four hundred years after he died before World War 1 saw tanks go into use during wartime. None of the early designs (including Leonardo's) ever reached the stage of being built.