The fall of the monarchy in 1931 and the instauration of the 2nd Republic inagurated a period in which Spain saw social differences increasing due to economic crisis and incompetent governments. By this time the successful October revolution had given all leftist parties a point of reference for violent change, while conservatives were scared of Revolution, being determined to combat the spread of marxism all through Europe.

The uprising of 18th July, 1936 gave both sides the needed excuse, and soon executions and repression was widespread as both parties settled scores. Militarily, the war was characterized by improvisation as both sides tried to gather sufficient forces to strike at enemy centers of power, but both sides were closely matched and soon the fact that the war was not going to be short was apparent.

Nationalist planning contemplated a converging attack on Madrid to take over a city that was both the seat of government and the major transport hub of the country, however the swift surrender of the rebels in Barcelona soon had military and militia columns advancing on Saragossa, therefore forcing the cancellation of one of planned axis of advance. Republican attacks on Cordoba and their success in stopping the castillian and navarrese columns on the mountains north of Madrid meant that the attack on Madrid could only be mounted from the south, however, this attack would be undertaken by the best nationalist troops. Advancing at full speed, and confronted only with militias through Extremadura and Castille, the Army of Africa was soon in the doors of Madrid, but then the tide turned, as the republicans mobilised and outside aid, both in men and materiel, started to arrive, and Nationalist momentum was lost when the desperate situation of a small goup under siege in the Alcazar of Toledo forced a detour to free them and secure a bridgehead on the Tajo. However, Franco hoped that the superior quality of his men would compensate the manpower and material superiority of the republican defenders, mainly militias, but fighting in cities was very different than in the open fields of castille, and the assault was repulsed with heavy losses, so by October, the nationalist amended their plans to surround Madrid first throught the north (in the battle of Corunna road) then through the south (the battle of Jarama), however both attempts were contained with heavy losses on both sides.

Now, a new phase opened as leaders on both sides reckoned that the war was going to be long, so both sides turned their attention to the periphery, the nationalist striking first when, with italian help, Queipo del Llano conquered Malaga in February.

Then, the Nationalist turned to the Republican north, which was isolated and disjointed by political infighting while the Republicans started to create a modern disciplined army from the different militias, which usually amounted to armed mobs.
Amid heavy fighting, the campaign in the north turned into a Republican disaster, so several attempts were made to distract Nationalists forces by attacks in Madrid (at Brunete), Aragon and Extremadura, with little success. Still, this battles gave the new Ejercito Popular confidence and experience.

Therefore, when the Nationalist turned their attention once more to the south in autumn 1937, they had a dangerous foe which was not ready to give up strategic initiative.