Britain and France didn't want Spain to fall under the Nationalists as it would strengthen Fascists power. They also didn't want it to be Soviet-backed as they thought communism was a threat to world peace.
Britain and France made a mutual policy to block international aid reaching Spain but couldn't stop Germany and Italy sending forces and supplies to the Nationalists.
As a result Hitler's position strengthened has he had another potential ally,General Franco, and his relationship with Mussolini strengthened.
The Nationlists won after Madrid was captured in March 1939