 Complete the following phrase:  As they had proven valuable during the Balkan Wars, the Serbian army used Chetniks in World War I in the same way, and while useful they suffered heavy losses. At the end of the Serbian campaign of 19141915, they withdrew with the army in the Great Retreat to Corfu and later fought on the Macedonian front. Montenegrin Chetniks also fought against the Austro-Hungarian occupation of that country. In late 1916, new Chetnik companies were being organised to fight in Bulgarian-occupied southeastern Serbia. Concerned about reprisals against a large-scale uprising, the Serbian army sent the veteran Chetnik leader Kosta Pećanac to prevent the outbreak. However, the Bulgarians started conscripting Serbs, and hundreds of men joined the Chetnik detachments. This resulted in the 1917 