== Background ==
On 22 March 1937, [[Francisco Franco|Franco]] decided to halt his offensive against Madrid and start an [[War in the North|offensive against the northern Republican held zone]]. The northern zone was politically divided and isolated from the central Republican zone. Furthermore, there was most of Spanish iron and coal and the chemical plants of Biscay.ref The Nationalists decided to start the occupation of the northern Republican zone with the conquest of the Biscay province.
== Opposing forces ==
The Nationalists had the Emilio Mola's Army of the North (55,000 men). The Nationalist attack started with the Navarrese Division, led by the General José Solchaga. This division had four brigades led by the Colonels [[Rafael García Valiño|Garcia Valiño]], Alonso Vega, Cayuela and Latorre (18,000 men) and the [[Flechas Negras Division|Black Arrow]] division (8,000 men with Italian officers). This force was established between and , on the border of the Biscay province. The Nationalist had also 200 guns, 120 aircraft, the battleship [[...|España]], the cruisers [[Spanish cruiser Canarias|Canarias]] and [[...|Almirante Cervera]] and the destroyer Velasco.ref
Opposing them, the Republicans had the Llano de la Encomienda's Army of the North, theoretically numbered 150,000, but there were no unity between the Basque Nationalists, the Asturians and the Santaderinos. The Basque Army in Biscay had 30,000 men (most of them Basque Nationalists and also Asturians). The Republicans also had 140 guns, 25-30 aircraft, two destroyers and three submarines.ref