The Battle of El Mazucu was fought between 6 and 22 September 1937, between the Republican and Nationalist armies during the Spanish Civil War as a part of the War in the North campaign. The Republican defence of El Mazucu and the surrounding mountains halted the Nationalist advance into eastern Asturias, despite their forces being outnumbered sevenfold. After weeks of intense fighting over extreme terrain the defenders were eventually overwhelmed, and the Nationalists were then able to link up with their forces advancing from León, leading to the fall of Gijón and the abandonment of Asturias, the last Republican province in North-West Spain. This battle was almost certainly the first use of carpet bombing against a military target. Prelude Following the fall of Bilbao and the defeat of the Republican forces defending Santander, the Republican stronghold of Asturias was isolated from the Republican armies in the south and east of Spain. The leader of the Nationalist forces surrounding Asturias, General Fidel Dávila, attacked from the south and from the east, expecting little resistance from the demoralized Republicans. The first republican line, along the Deva River, was soon overrun, and the town of Llanes fell on 5 September 1937. However, the routes the Nationalists then had to take were commanded by the limestone walls of the Sierra de Cuera on the north of the front and the Deva Gorge to the south. The Nationalists had to clear the defenders from these mountains in order to advance, and to do that they planned a pincer movement moving southwest from Llanes and west, along Cares river, from Panes towards Cabrales. On both fronts, the rugged terrain and stiff Republican resistance halted the advance. It was then clear that the mountains of the Sierra de Cuera were vital to the defence of Asturias, and the key to the Sierra de Cuera was the pass of El Mazucu. The combatants The Nationalist forces comprised four Navarrese Brigades (33,000 men), under the command of General José Solchaga Zala (Solchaga) in Llanes, with 15 artillery batteries and strong air support (including the German Legión Cóndor, partly based on the Cue Aerodrome). The pass of El Mazucu is only five km from the sea, and so the cruiser Almirante Cervera was also able to use its 6 in. guns in the action. The Asturian and some Basque and Santander forces (Republicans) comprised three weakened brigades (fewer than 5,000