In 1937, Nelson immediately tried to join the Abraham Lincoln Brigade but had to stay in Pennsylvania, where he agitated coal miners. After the severe setback at Jarama, Nelson, Joe Dallet and 23 others were allowed to fight in Spain. At first arrested by the French, they reached Spain climbing the Pyrenees mountains and met the International Brigades at Albacete in May 1937. Nelson started the war as political commissar of the Lincoln Battalion. After heavy losses at Brunete, the Abraham Lincoln Battalion and the George Washington Battalion were merged into the Lincoln–Washington Battalion. Mirko Markovicz, a Yugoslav-American, was appointed as commander of the Lincoln–Washington Battalion and Nelson became his political commissar. In August 1937 the American forces were reorganized. Nelson was promoted to XV International Brigade commissarand Robert Hale Merriman became brigade chief of staff. Hans Amlie became commander of the Lincoln–Washington Battalion. In the fighting at Belchite, which started very badly, only two soldiers of 22 survived the first attempt to take the church of the town. Nelson then led his men in a successful diversionary attack; and Amlie's men entered the fortified town. Nelson was wounded, shot in the face and leg, and Merriman and Amlie received head wounds. After recovering in Valencia, Nelson was given the task to escort prominent visitors (such as John Bernard, Dorothy Parker and Ernest Hemingway).