The history indicated that in the Meuse valley between Dinant and Liège in Belguim, when the river became frozen and the ice was so thick and they cannot get fish for their meals. Instead, in these times, they would bring the potatoes, slice them in long thin shape and fry them for making food.
But no matter how the history has said about fried potatoes, it seems like the French was the one who brought fries into America and Britain. And made it became popular fast food in present days.