from Wikipedia on the origin of Belgian fries:
Belgian journalist Jo Gérard has claimed that a family manuscript dated 1781 recounts that potatoes were deep-fried prior to 1680 in what was then the Spanish Netherlands and is now present-day Belgium, in the Meuse valley: "The inhabitants of Namur, Andenne, and Dinant, had the custom of fishing in the Meuse for small fish and frying, especially among the poor, but when the river was frozen and fishing became hazardous, they cut potatoes in the form of small fish and put them in a fryer like those here". Gérard has not produced the manuscript supporting this claim which, even if true, is unrelated to the later history of the French fry, as the potato did not arrive in the region until around 1735; also, given the economic conditions of the 18th century: "it is absolutely unthinkable that a peasant could have consecrated large quantities of fat for cooking potatoes. At most they were sautéed in a pan...." Some Belgians believe that the term "French" was introduced when American soldiers arrived in Belgium during World War I, and consequently tasted Belgian fries. They supposedly called them "French," as it was the official language of the Belgian Army at that time. At this time French fries were growing popular. However, in the south of Netherlands, bordering Belgium, they were, and still are, called Vlaamse Frieten or "Flemish fries."
"Pommes frites," "frites"(French) or "frieten" (Dutch) became the national snack and a substantial part of several national dishes.