The Republicans sent a force of militiamen and soldiers from Madrid to stop the advance, with most of the fighting taking place at the Somosierra pass in the last week of July and in early August.

Mola realised that it would be difficult for the coup to succeed by itself in the capital Madrid under the command of General Joaquín Fanjul, so it was planned that a column from the north would march on the city to support the uprising.

At midnight on 21 July, another Nationalist column made up of two or three hundred men led by Colonel Ricardo Serrador Santés left Valladolid "amid scenes of indescribable enthusiasm."

General José Sanjurjo continued with the mission of occupying Alto del León, the other important pass in the Sierra de Guadarrama, located to the west of Somosierra.

[1] A Republican column under the command of Colonel Mangada left Madrid in the direction of Ávila to try to isolate the Nationalist forces occupying Alto del León from the rear.

In his advance, Mangada conquered several towns in which the Civil Guard had joined the coup, but did not go beyond Navalperal de Pinares, fearful of losing communication with Madrid and being isolated.

Lisardo Doval, the commander of the Civil Guard known for having led the brutal repression that followed the defeat of the Asturian miners' strike of 1934, tried but failed to stop Mangada's advance.

These columns were joined by the CNT, a powerful anarchist trade union which had formed its own militias, and included prominent leaders from Madrid such as Cipriano Mera and Teodoro Mora.

[5] The fighting in the Sierra de Guadarrama was fierce and led to prisoners of war being shot on both sides, but the Republicans were able to halt the Nationalist advance on Madrid from the north.