On 9 February 1946, the 26th plenary session of the UN General Assembly adopted a resolution regulating relations between UN members and Francoist Spain. The document recommended that countries should be guided in their relations with Spain by the decisions of the Potsdam Conference (“Big Three” leaders) and the San Francisco Conference where the UN Charter was approved.

At the Potsdam Conference, the Allies stated that they would not support Francisco Franco admission to the UN, “which, having been founded with the support of the Axis powers, in view of its origins, its nature, its record and its close association with the aggressor States, does not possess the necessary qualifications to justify its admission.”

In San Francisco UN founding members adopted a resolution restricting membership to “states whose regimes have been installed with the help of armed forces of countries which have fought against the United Nations.”

The Franco regime was established as a result of a Spanish nationalist uprising in 1936, supported by Hitler's Germany. Franco's rule would last until 1975. Diplomatic sanctions against Spain would begin to be lifted in the 1950s and Spain would join the United Nations in 1955.

Sources:

Relations of Members of the United Nations with Spain [1946] UNGA 18; A/RES/32 (I) dated 9 February 1946