The Spanish Civil War (1936 - 1939) saw right-wing Nationalists, led by General Franco, fight against the left-wing Republicans. Germany and Italy supported the Nationalists with planes, tanks and weapons. The USSR supported the Republican forces.
The British government did not want the Spanish Civil War to escalate into a Europe-wide conflict and so signed up to the Non-Intervention Committee.
For some in the British political establishment, Spain confirmed their doubts about Britain’s policy of appeasement. Anthony Eden, who had been a supporter of the government’s non-aggressive approach, began to believe that Britain should take a firmer stance with Europe’s fascist dictators, and resigning his office in 1938.
On 26 April 1937, Nazi German and Italian bombers attacked the Basque city of Guernica. Over the course of three hours, they destroyed three-quarters of the ancient town, killing and wounding hundreds.
The bombing of Guernica heightened fears of the consequences of any war. The British Prime Minister at the time, Stanley Baldwin, stated:
the bomber will always get through
It was believed aircraft would have a devastating impact. The British Government expected around 600,000 casualties in the first few days of any war.
The lack of air defences over Britain and a shortage of air raid shelters encouraged support for appeasement.