First Period: September 1939 - June 1940 - The Spanish continuation war
When Germany invaded Poland, two days later UK and France waged war on Germany.
One day later both sides allied with their respective supporters and, in the case of the Republicans this resulted to be in more supplies, specially in better airplanes and better trained pilots. That's how the
International Air Brigades were founded, however they only had some Gloster Gladiator MK.I to use:
The Gloster Gladiator probed to be an excellent aircraft, specially against the Italian Fiat CR.32 of the nationalist side.
The republicans expected the Frenchs to invade the Iberian peninsula by the nationalist side, however, they were more concerned about the Franco-German border and they didn't send many help, apart from opening their ports to Republican merchants in all their ports, and supplying some airplanes, like the Morane Saulnier MS.406, which, was not seen as a very good airplane:
The nationalists, wanting to conquest Gibraltar (they were at war with the UK now that they were allied with Germany), launched a rather catastrophic assault to conquest it. Obviously they couldn't conquest it and they gave up on their efforts as they realized that they didn't have the ways to take the rock.
After Poland fell, the UK had prepared some Hurricanes to be sent to Poland, however they couldn't be released and they were sent to the Republican side where they played a brief, but very important role because they proved to be a very nice enemies for the nationalists:
The British also sold some Handley Page Hampden which were used in the central front, in Guadalajara, just east of Madrid:
Note: The inscription on this airplane reads "Bombas para los facciosos" which translates as "Bombs for the fascists"
Also, on this period of the war, in October 1939 to be more precise, the French 3eme division Marocaine (3rd Moroccan division) supported by the 2e régiment étranger de cavalerie (belonging to the foreign legion) invaded the
Spanish protectorate in Morocco (controlled by the nationalist since the beginning of the war) encountering almost no resistance because most of the nationalist units of that area were fighting in the peninsula.
The French government tried to sell this as an important victory, however most of the French public opinion wondered if such movement was necessary and it created some unrest among the French generals.
Note: I know that there was a drawing here, but it's more suitable for the next period, so it will be reposted when i've finished it.