First World War: 1916 (Feb. 24) Stampless cover from French Corsica sent to a Belgian internee in the Netherlands canceled by 'Ajaccio Corse' postmark. French Military Censor's tape at left tied on both sides by oval censor cachet

Note that the addressee was not a Prisoner of War. Corsica was part of France and therefore, like Belgium, was part of the Allies fighting against the Central Powers. 

The Netherlands maintained its neutrality throughout the war. The Belgian addressee was held in a Dutch internment camp, as were all belligerents who entered Dutch or Swiss territory. 

Out of Corsica's population of 300,000, a total of 50,000 men served in the French military during the war, the highest proportion of any Allied area. Accordingly, Corsica suffered tremendous casualties, leading to a severe postwar economic recession and large emigration from the island. 

Extremely rare usage. Probably unique.

Estimate $275