The unsuccessful capture of Madrid provided an important lesson to Franco for the future of war: Continue with the capture of a city as heavily fortified as the nation's capital, did not seem very sensible. The best time was cementing his power especially in the northern province, which would give the opportunity to complete the circle, which prepared a long time. The National High Command, determined to end the last pockets of Republicans in the Northern provinces, turned his attention towards the conquest of Bilbao, Santander and mainly Asturias, where Republicans had become strong. On March 31, 1937 began the offensive. The Nationalists brought the best of its offensive in the operation. If they could secure Norte, their hands would be free to regroup their forces around Madrid. Preparations for the expedition of the North were meticulously.
Franco was supported by the elite division (the division Navarra) consisting of four brigades with 18,000 men. At the same time, received the support of the brigade "Arrows Black, Spanish battalion composed of 8000 led by Italian officers. The air dominance was guaranteed by Italian and German aircraft, and sea blockade by the national team with the cruiser Canarias "in front. The conquest of the North would be supervised by the general plain of the Commandery, who as a first task, was designed to ensure the conquest of the Basque country (northern Spain, bordering France). It began, the famous campaign of Biscay. Republicans, whose possessions outside the Spanish center hung in the balance, had tried to regroup while attacking .