 In December 1948, the Dutch army launched Operation "Kraai" (Dutch for "Crow"), swiftly captured Yogyakarta, and imprisoned Sukarno and other most nationalist leaders (see Politionele acties and Operation Kraai).During the assault upon the provisional capital, the nationalist rebels released Princen from their prison and gave him the chance to enlist in the Tentara Nasional Indonesia (TNI, Indonesian Republican Forces).When he joined them, the pro-independence forces' fortunes seemed at their nadir, with their political leadership captured and most of the territory of Indonesia under a re-established Dutch military rule.