The International Brigades were a military unit of the Spanish Republican Army, organized by the Komintern, and made up of around 35.000 anti-fascist civil volunteers from around the world. Combatants from up to 54 countries enlisted to fight fascism in the Spanish Civil War (1936-1939). Despite the military inexperience of most of its members, the International Brigades became the shock force and the backbone of the Republican Army. Thus, they fought in the most important battles of the war: the defense of Madrid (November – December 1936), the Corunna road (January 1937), Jarama (February 1937), Guadalajara (March 1937), La Granja (May – June 1937), Brunete (July 1937), Belchite (August – September 1937), Teruel (December 1937 – February 1938), the fall and retreat of Aragon (March – April 1938) and they also participated in the offensive and subsequent Battle of the Ebro (July – November 1938). The International Brigades were withdrawn from the front on September 23, 1938 and subsequently demobilized following the Munich Agreement (September 29-30, 1938). They briefly returned to combat as an international group in the end of January and during February 1939, in defense of the last Catalan territory loyal to the Spanish Republic and after having crossed the French border on February 9, 1939. In the summer of 1938, the International Brigades were more of a theoretical force than a real one, since only a third of its members were international volunteers. This was as a result of the bloody attacks suffered by these combatants during the war and the decrease in the arrival of new volunteers, being the remaining two thirds represented by Catalan and Spanish fighters.
The forces of the International Brigades that fought in the Ascó sector remained integrated in the 35th International Division: the XIth International Brigade Thäelmann, the XIIIth International Brigade Dombrowski and the XVth International Brigade Lincoln.