. . were invented in Belgium.

Returning WWI US soldiers demanded them. They had been stationed in Belgium, but thought they were still in France. French French Fries are round, not stick shaped.

Perhaps the French just finally decided to go along with whatever everyone else is doing.

The first mention in France is from 1775 (round slices). That they were round is also attested by Thomas Jefferson (1802) and E.Warren (England 1856). The Belgians were the first to cut them in a stick shape.

French Fries were round in the United State, in the French manner, until after WWI.

The American Potato Chip was invented by an angry chef in Saratoga New York, when a customer rejected his (round) French Fries as too thick. He cut a batch as thin as he could (round), fried them up and sent them out. The customer loved them - and the Saratoga Chip was born. For a long time, potato chip slicers were called "Saratoga slicers).