During the First World War, Joseph De Cuyper served as a medical assistant on the Yser Front. He went through several postings: from 1DA 22nd Line, to the 2nd Line, then the 1st and 7th Artillery, and finally the 2nd Line, 2nd Battalion.
During the liberation offensive, Joseph took part in the battle of Houthulst - Klerken. The Belgian army advanced to Zarren, but ran into heavy resistance.
On 30 September 1918, in Zarren, Joseph was killed by a shell explosion right next to the miller Couchez’s house. His friend, artillery lieutenant Corneel Heymans – who would later win the Nobel Prize in Medicine in 1938 – remembered their last conversation just before the fatal blast.
He rests in the family grave at the central cemetery in Bruges.
His diary Journal de Campagne 1914-1917. Oorlogsdagboek van een hulpdokter bij het Belgisch leger, which recounts life in the trenches, was published in 1968.