The Basque people in Spain generally inhabit four provinces: Navarre, Álava, Gipuzkoa and Biscay.

[1] The Spanish Nationalist troops gained Gipuzkoa early in the war with the fall of Irún in August and San Sebastián on 13 September 1936,[2] isolating the Basque Country and the zone held by the Northern Republicans from the French border.

[3] It was quite an antiquated concept, akin to First World War fortifications, and so was vulnerable to modern warfare and weapons, such as aircraft and artillery.

[4][5] The ring was breached by an infantry assault supported by heavy air and artillery bombardment (150 guns and 70 bombers).

On 18 June, General Ulibarri withdrew his remaining troops from Bilbao, and the Nationalists occupied the city on the following day.