
In the 1930s, Spain was a deeply divided country that was politically torn between right-wing Nationalist and left-wing Republican parties. The Nationalist party was made up of monarchists, landowners, employers, the Roman Catholic Church and the army. The Republicans consisted of the workers, the trade unions, socialists and peasants.
Economically, the country had been deeply hit by the Great Depression after the Wall Street Crash. Partly due to this turmoil, in 1929 the military dictatorship that had ruled Spain since 1923 collapsed. In 1931 the King abdicated after the Republicans came to power.
There followed a period where the two political rivals had periods in power as the elected government. The country was so divided and unstable that in 1936 the army rebelled and forcibly removed the Republicans from power. Civil war ensued.
If Spain fell to the Nationalists, France would be surrounded by Fascist powers (Germany and Italy). If France was invaded by Fascist nations, the alliances between other anti-Fascist nations would be weakened. In effect, there would be one less nation to resist Fascist plans to expand their borders - one less army to stand up to them.
Spain also had strategic naval bases on the Mediterranean Sea and Atlantic Ocean that could be used by the Fascists to control shipping and for setting up submarine bases. These could be used to put military and economic pressure on other European nations.