Why did Franco not attack Gibraltar during the Second World War?

Why did Franco not make a serious attempt to seize Gibraltar during the Second World War?

Spain emerged from civil war militarily and economically weak, while Gibraltar was a heavily armed fortress that could not be taken quickly without risking war with Britain and likely the United States.

Franco needed trade and internal recovery after the Spanish Civil War, and benevolent neutrality toward Germany offered many benefits without the cost of formally entering the war. By the time Spain was more stable, American alignment was clear and Gibraltar remained a prepared defensive position with a large garrison and heavy coastal artillery. Tanks still had to approach over a narrow isthmus under anti-tank and naval-calibre guns, while the Spanish Navy had also suffered in the civil war. German support would have required a long supply line through France, while Spain offered limited natural resources and industry and still needed trained troops at home. Franco could also see how Mussolini's alliance with Hitler had reduced Italy to the junior role and had little incentive to accept the same loss of autonomy. The conclusion is that Franco, whatever his politics, was competent enough to avoid treating Gibraltar as an easy prize.