JHE
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MI6 spent the present-day equivalent of more than $200m bribing senior Spanish military officers, ship owners and other agents to keep Spain out of the second world war, files released today disclose.
More and more money was delivered, mainly via a Swiss bank account in New York, as Sir Samuel Hoare, Britain's ambassador in Madrid, warned London that unless it was paid, there was a real and immediate danger of Spain abandoning its neutrality and of Franco joining forces with Nazi Germany.
http://www.guardian.co.uk/uk/2013/may/23/mi6-spain-200m-bribes-ww2
A very interesting revelation, but whether this dosh really had any effect is another matter. I don't know.
Paul Preston's account of Spain not joining, in his biography of Franco, is that though Franco was for a long time convinced that the Axis would win and was keen to join in, he never got the deal he wanted. He wanted an agreement that, following an Axis victory, Spain would have a free hand to carve out a new north African empire (taking territory from France, of course). Hitler was unimpressed by the squeaky little Galician general and didn't offer what was demanded. Franco was also worried (groundlessly or not, I really don't know) that if Spain, or Germany with Spanish help, took Gibraltar, Britain would attack and seize some part of the Canary Islands.