Why did the Prix Lupin not take place from 1915 to 1918?

Context: The event was established in 1855, and it was originally called the Prix de l'Empereur. It was initially held at the Champ de Mars, and was transferred to Longchamp in 1857. It was cancelled due to the Franco-Prussian War in 1871, and was renamed the Grande Poule des Produits in 1872.

The race was one of several trials for the Prix du Jockey Club collectively known as the Poules des Produits. The others were the Prix Daru, the Prix Hocquart, the Prix Noailles and the Prix Greffulhe. Unlike those races, the Grande Poule des Produits had no restrictions based on the nationality of a horse's sire or dam.

The event was renamed in memory of Auguste Lupin, a successful owner-breeder, in 1896.

The Prix Lupin was abandoned throughout World War I, with no running from 1915 to 1918. It was cancelled once during World War II, in 1940. It was run at Le Tremblay over 2,150 metres in 1943, and at Maisons-Laffitte in 1944.

The present system of race grading was introduced in 1971, and the Prix Lupin was classed at Group 1 level.

The race was last run in 2004. It was discontinued after France Galop restructured its Group 1 programme for three-year-olds in 2005.

Answer: Due to the first World War