Lasting from July 17th 1936 until April 1st 1939, the Spanish Civil War was triggered by a military uprising led by General Franco.

With Italian and German support, his Nationalist forces gradually defeated the incumbent government of the republic and the left-wing groups aligned to it. With casualties sometimes estimated as reaching one million, there were brutal tactics on both sides and with the end of the war came terrible reprisals against those who supported the Republicans, while Spain itself remained a dictatorship under Franco's until his death in 1975.

Republican reprisals against the Catholic Church, which supported the uprising, meant that the Irish clergy urged its people to support Franco and his cause. Some 750 men, led by Eoin O'Duffy, formed the Bandera who travelled in support of Franco. Meanwhile, more than 300 Irish fought with the International Brigade, which comprised men from other countries, including Britain, the US and France.