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.

Euzko Gudarostea was the name of the army commanded by the Basque Government during the Spanish Civil War. It was formed by Basque nationalists, socialists, communists, anarchists and republicans under the direction of lehendakari José Antonio Aguirre and coordinating with the army of the Second Spanish Republic. It fought the troops of Francisco Franco during 1936 and 1937. It surrendered to the Italian Corpo Truppe Volontarie at Santoña, while the rest of the Republican army kept fighting until 1939. This event is called the Santoña Agreement, Pact of Santoña, or Treason of Santoña by some Spanish leftists.