War in the North The War in the North , in the Spanish Civil War was the campaign in which the Nationalist forces defeated and occupied the parts of northern Spain that remained loyal to the Republican government . The campaign included several separate battles . The Biscay Campaign resulted in the loss of the part of the Basque Country still held by the Republic and Bilbao , the greatest Spanish industrial center . This part of the campaign saw the Bombing of Guernica and Durango . The Battle of Santander caused the loss of the province of Santander in Cantabrian Castile for the Republic . The Battle of El Mazuco lead to the capture of the Republican - controlled part of Asturias and the fall of Gijón , the Republic ' s last stronghold in the North , to the Nationalists . The campaign ended on October 21 , 1937 with a decisive and total Nationalist victory . Introduction . With the fall of Irun and then San Sebastián , in September 23 , 1936 , the Nationalists under Franco campaign in Gipuzkoa cut off the Republican controlled areas in northern Spain from the border with France . This area had been already isolated from the rest of Spain by Nationalist controls at the beginning of the war . This area was very attractive to the Nationalists because of the industrial production of Biscay and the mineral resources of Asturias . To conquer and control this area would be profitable through its valuable resources , would expel Republican forces and concentrate large numbers of Nationalist troops to dictate a two - front war . Franco realized that Madrid , the capital , was not going to be conquered quickly . Basque ' s resources of iron , coal , steel and chemicals were a tempting target . The Republican north was also politically divided and weakened by struggles between Basque nationalist s and leftists . Furthermore its major supplies came by sea . Franco wisely ordered to his commanders in the Madrid front to go on the defensive and send all available resources to the north . Beginning of the campaign . Emilio Mola was in command of the start of this campaign , beginning on March 31 , 1937 ; however , Mola , died in an airplane crash , June 3 , 1937 . The Nationalists began the attack with 50 , 000 men of the 61st Navarre Division under General José Solchaga . The Republican Army of the North was commanded by General Francisco Llano de la Encomienda . This was the beginning of the Biscay Campaign . The Nationalist offensive started on March 31 , and the same day the " Legion Condor " bombed the town of Durango , there were 250 civilian deaths . The Navarrase troops attacked the town of Ochandiano and on April 4 occupied it , after heavy combats . 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 , but some British ships entered Bilbao . On April 20 , the Nationalists continued their offensive and occupied Elgeta after a heavy artillery bombardment . The same day the " Legion Condor " bombed Guernica . The Basques retreated to the Iron Belt line and on April 30 , the Italians occupied Bermeo , but the Nationalist battleship Jaime I , was sunk by a mine . Then the Republican government decided to send 50 aircraft to Bilbao and launched two offensives against Huesca and Segovia in order to stop the Nationalist advance , but both failed . On June 3 Mola died and was replaced by Davila . On June 12 the Nationalist started the assault of the Iron Belt and after heavy aerial and artillery bombings entered in Bilbao in June 19 . Battle of Santander . After the fall of Bilbao , the Republican government decided to launch an offensive against Brunete in order to stop the Nationalist offensive in the north on July 6 , but by July 25 the offensive had ended . The morale of the Republican troops in Cantabria was low and the Basque soldiers did not want to carry on the fight . On August 14 , the Nationalists launched their offensive against Cantabria , with the 90 , 000 men ( 25 , 000 Italians ) and 200 aircraft of the Army of the North . On August 17 , the Italians occupied the " El Escudo " Pass and encircled 22 Republican battalions at Campoo ( Cantabria ) . On August 24 , the Basque troops surrendered to the Italians at Santoña and the Republican troops fled from Santander . On August 26 , the Italians occupied Santander and by September 1 , the Nationalist had occupied almost all Cantabria . The Nationalist captured 60 , 000 prisoners , the greatest victory of the war . Asturias Campaign . After the failed republican offensive against Zaragoza , the Nationalist decided to continue their offensive against Asturias . The Nationalist had an overwhelming numerical ( 90 , 000 men against 45 , 000 ) and material ( more than 200 aircraft against 35 ) superiority , but the Republican Army in Asturias was better organized than in Santander and the difficult terrain provided excellent defensive positions . The Navarrese troops ( 30 , 000 ) , led by Solchaga , and supported by the " Legion Condor " , managed to occupy the El Mazuco Pass , held by 5 , 000 Republican soldiers , only after 33 day of bloody combats . On October 14 the Nationalists broke the Republican front , and on October 17 the Republican government ordered to start the evacuation of Asturias , but Nationalist ships were blockading the Asturian ports and only a few military commanders ( Adolfo Prada , Galan , Belarmino Tomas ) managed to escape . By October 21 the Nationalists occupied Gijón and completed the conquest of the northern zone . Aftermath . With the conquest of the North , the Nationalist , controlled the 36 per cent of the Spanish industrial production , 60 per cent of the coal and all the steel production . Furthermore , more than 100 , 000 Republican prisoners were forced to join the Nationalist army or sent to labour battalions . The Republic had lost the Army of the North ( more than 200 , 000 soldiers ) and by then a complete military victory of the Republic in the war became impossible . Then Franco , decided to start a new offensive against Madrid , but Vicente Rojo , the leader of the Republican Army , launched a diversionary offensive in Aragon , the Battle of Teruel .