The Nationalists turned to destroying the Republican army enclave along the Bay of Biscay in the summer, 1937. Sperrle moved his headquarters to Vitoria to lead his small force of 62 aircraft. The Biscay Campaign began on 31 March with the Bombing of Durango. Spanish nationalists reported Republican army movements in the town, but before the bombers arrived they withdrew, leaving the bombers to devastate the town and kill 250 civilians. Sperrle protested to the Nationalists about the "waste of resources", but his own authorised attacks on the town's arms-factory were inaccurate and did just as much to damage the Legion's image. Thereafter Sperrle supported the battle for Otxandio. The Nationalists refused to cooperate in an encirclement operation allowing the defenders to retreat toward Guernica. In a farcical moment Sperrle was arrested on a visit to the front by Nationalist forces as a suspected spy.