The Nationalists launched a number of offensives into parts of the Basque Country, Santander (Cantabria), and Asturias that formed an enclave loyal to the Republican government.

The Biscay Campaign from March to July saw the bombing of Guernica and Durango and resulted in the Republicans losing the Basque Country after the Battle of Bilbao.

In July 1936, upon the Nationalist takeover of Navarre, their leader General Emilio Mola had announced a war of annihilation against the Second Spanish Republic and no mercy to any dissent.

By late August, the Requeté, a pro-Nationalist Carlist militia from Navarre, advanced towards Irun in the Basque Country with a mission to cut off Republican forces in Gipuzkoa from the France–Spain border.

[4] In September, after the fall of Irun and then of San Sebastián, the Nationalists led by Francisco Franco launched a campaign in Gipuzkoa and cut off the Republican-controlled areas in northern Spain from the border with France.

Franco realised that the capital Madrid was not going to be conquered quickly after a number of Nationalist offensives on the city and the surrounding area had failed.

The Basques retreated to Bilbao's Iron Ring, and on 30 April, the Italian Corps of Volunteer Troops occupied Bermeo, but the Nationalist battleship España was sunk by a mine.

The Nationalists had overwhelming numerical and material superiority, with 90,000 men against 45,000 and more than 200 aircraft against 35, but the Republican Army in Asturias was better organised than in Santander and the difficult mountainous terrain would provide excellent defensive positions.

The El Mazuco valley and the critical nearby mountains (Peña Blanca and Pico Turbina), which were held by 5,000 Republican soldiers, fell to the Nationalists only after 33 days of bloody combat.

However, Nationalist ships were blockading the Asturian ports, and only the few military commanders Adolfo Prada, Francisco Galán, and Belarmino Tomas managed to escape.