Hitler had sought Franco's permission to cross Spain in order to attack Gibraltar. Negotiations were tough and exceedingly slow at the end of which Franco had hardly offered up anything of substance to Hitler. Hitler was asked if there would be another round of negotiations, to which Hitler replied,"I'd rather have my eyeteeth pulled."
According to Winston Churchill, “before the Torch operation, i.e. before the landing of the forces of the United States and Britain in the North of Africa. (...) I have to say that I shall take it that Spain paid then a service not only to the United Kingdom, the British Empire and the Commonwealth, but the cause of the United Nations. For this reason do not sympathize with those who believe intelligent, and even funny, insult and offend the Government of Spain in any occasion”(Winston Churchill, British Prime Minister during World War II.)
In Winston Churchill’s “The Second World War”, Vol.2: Their Finest Hour Pages 519-530, the British Prime Minister wrote that last possibility of Hitler’s triumph was foiled by Franco:
“It is fashionable at the present time to dwell on the vices of General Franco and I am glad therefore to place on record this testimony of duplicity... I shall presently record even greater services which… General Franco rendered the Allied cause.”
If Franco had allowed Hitler to take over Gibraltar, the African side of the Strait under Spanish rule, and the Nazis would have controlled the access to the Mediterranean making impossible for the Allies to win the war in north Africa or to invade Italy.