'Chilblain' was a US special forces reconnaissance operation by the Office of Strategic Services in German-occupied Yugoslavia (August/22 September 1944).

The objectives of the Office of Strategic Services in the Yugoslav theatre were to engage in as many offensive missions as possible to the Dalmatian coastal islands, where there were concentrations of German forces, and to undertake reconnaissance and intelligence activities.

Troops of Company C destined for Yugoslav operations started arriving at Bari in south-eastern Italy, the OSS rear-echelon base, during January 1944. As more units came from the USA they transferred first to the vicinity of Manfredonia for additional training and next to Torre a Mare, a larger facility. Troops then were sent to the Dalmatian coastal island of Vis, where the first men arrived on 20 January. At this base for Yugoslav missions they became part of the Allied garrison, which included British commandos, the Raider Support Regiment and other British units, as well as partisans. Also available was naval and air support. The total of Allied troops on the island was several thousands.

Between January and June 1944, reconnaissance patrols of the Office of Strategic Services reached 10 Yugoslav islands and the mainland, several visits being made to each island each month. Officers and men in each party varied up to eight and were maintained on the island up to two weeks, and their objectives included liaison with the partisans of Marshal Josip Broz Tito, ground reconnaissance, and daily reports by radio to an advanced headquarters on the island of Vis. Reports were sent to British brigade ground forces, the Royal Navy, Royal Air Force, the partisans and Allied headquarters in Italy.

The reconnaissance parties took with them full equipment, weapons and rations, and the partisan forces provided transportation in small fishing boats and schooners, though at times British and US naval vessels aided the effort. Trips took two nights and were made hazardous by storms and the presence of German vessels. The accomplishment of their mission meant that the US troops had to live with the partisans, and on land the primary hazards were German patrols, extensive stormy weather and the delayed arrival of relief parties. However, no casualties resulted from the transportation missions.

After several attempts to transport the advance party to the Biokov area of the Yugoslav mainland, a unit under the command of Captain L. B. Prus had got as far as the mainland coast in 'Chilblain' before a German gunboat opened fire and forced the partisan craft to withdraw. One man, T/5 A. Radac, was left behind, found the local partisan headquarters and was returned to Vis on 12 September.

Further consideration of the operation was lost in the political policies of the partisan high command, and the last OSS unit left Vis on 22 September.