Biscay Campaign The Biscay Campaign ( ) was an offensive of the Spanish Civil War which lasted from 31 March to 1 July 1937 . 50 , 000 men of the Eusko Gudarostea met 65 , 000 men of the insurgent forces . After heavy combats the Nationalist forces with a crushing material superiority managed to occupy the city of Bilbao and the Biscay province . Background . On 22 March 1937 , Franco decided to halt his offensive against Madrid and start an 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 . 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 Garcia Valiño , Alonso Vega , Cayuela and Latorre ( 18 , 000 men ) and the Black Arrow division ( 8 , 000 men with Italian officers ) . This force was established between Vergara and Villareal , on the border of the Biscay province . The Nationalist had also 200 guns , 120 aircraft , the battleship " España " , the cruisers " Canarias " and " Almirante Cervera " and the destroyer " Velasco " . 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 . The Offensive . The Nationalist offensive started on March 31 , and Mola threatened to bomb the Basque cities and industries : " I have decided to terminate rapidly the war in the north : those not guilty of assassinations and who surrender their arms will have their lives and property spared . But , if submission is not immediate , I will raze all Vizcaya to the ground , beginning with the industries of war . " . The same day the Legion Condor bombed the town of Durango , there were 250 civilian deaths . On April 1 , the Colonel Camilo Alonso Vega captured the mountains of Maroto , Albertia and Jarindo , and the Navarrase troops attacked the town of Ochandiano and encircled the Basque troops in it . The Navarrese occupied the town on April 4 , after heavy combats and aerial bombardment . The Basque troops left 400 deaths and 600 prisoners . Then Mola , decided to stop the advance due bad weather . On April 6 , the Nationalist government in Burgos announced the blockade of the Basque ports . The British government said that the blockade was effective and warned British ship not to go to Bilbao . Nevertheless , British merchants broke the blockade and entered in Basque ports . On April 20 , the Nationalists continued their offensive after a heavy artillery bombardment . The Basque troops led by the major Pablo Belderraín tried to resist but the 1st Navarrese Brigade led by the colonel Garcia Valiño broke the front and occupied Elgeta . The same day the Legion Condor bombed Guernica . The Basques retreated to the Iron Belt line and the " Legion Condor " bombed the roads and woods with incendiary bombs . Then , the " Lendakari " , Jósé Antonio Aguirre , decided to assume the command of the Basque troops , due the incompetence of Llano de la Encomienda . On April 30 , the Italians occupied Bermeo , but the Nationalist battleship " Jaime I " , was sunk by a mine . The fall of Bilbao . The Republican government tried to send fighters to the Basque country across France , but the French government returned the aircraft after confiscate its machine guns . The commander of the " FARE " , Hidalgo de Cisneros , decided to send 50 fighters and bombers to the Basque country , across the Nationalist held territory , 45 reached Bilbao . Meantime , the bad weather stopped again the Nationalist offensive , a new shipment of weapons ( 55 antiaircraft guns , 30 cannons and two squadrons of " Chatos " ) , reached Bilbao , and the General Gamir was sent to Biscay in order to organize the defense of Bilbao and to replace Llano de la Encomienda . The Republican government launched two offensives in Segovia and Huesca to halt the Nationalist offensive against Bilbao but both failed . Mola died on June 3 and was replaced by General Davila . On June 11 the Nationalist troops reached the Iron Belt and on June 12 after a heavy aerial and artillery bombing ( 150 guns and 70 bombers ) the Nationalist troops assaulted it . A Basque deserter , the Major Goicoechea gave to the Nationalist the plans of the Iron Belt . The Nationalist attacked in the Mount Urcullu and broke the Basque lines . On June 14 the Basque government left Bilbao , on June 18 the Basque troops was ordered to leave the city and by June 19 the Nationalist conquered the city . Aftermath . The Basque autonomy was abolished and the Basque tongue forbidden . Biscay had the only factory in Spain capable of the manufacture of artillery shells and half of the Spanish production of explosives .