In July 1936, on the outbreak of the Spanish Civil War, he urged the Republican Congress to send fighters to the International Brigades. He himself returned to Dublin with the intention of organising such a force. By the end of 1936, he had gone again to London and joined the Brigades. He reached Spain on 7 January 1937 and at Albacete, met up with an Irish contingent, led by Frank Ryan, known as the Connolly Column who had come to Spain to fight on the Republican side. Donnelly and his comrades were attached to the American Abraham Lincoln Battalion. On 15 February, after receiving only rudimentary military training, the Abraham Lincoln battalion was thrown into the battle of Jarama, near Madrid. Donnelly reached the front on 23 February, where he was promoted to the rank of field commander. On 27 February his unit were sent on a frontal assault on the Nationalist positions on a hill named Pingarron. Donnelly and his unit were pinned down by machine gun fire all day. In the evening, the Nationalists launched a counter-attack.