after the failure to take madrid in november 1936 and the failure to reduce the morale of the city's population through aerial bombing, franco decided to encircle the city from the north-west in order to cut off water and electricity supplies from the sierra de guadarrama. franco's nationalists, led by varela, concentrated a force of 3,000 men supported by heavy artillery including ju 52 bombers. the defending republican army had one brigade.