The Battle of the Ebro was the longest and largest battle of the Spanish Civil War and the greatest, in terms of manpower, logistics and material ever fought on Spanish soil. It took place between July and November 1938, with fighting mainly concentrated in two areas on the lower course of the Ebro River, the Terra Alta comarca of Catalonia, and the Auts area close to Fayón (Faió) in the lower Matarranya, Eastern Lower Aragon. These sparsely populated areas saw the largest array of armies in the war. The battle was disastrous for the Second Spanish Republic, with tens of thousands left dead or wounded and little effect on the advance of the Nationalists.

The Ebro is a river of the north and northeast of the Iberian Peninsula, in Spain. It rises in Cantabria and flows 930 kilometres (580 mi), almost entirely in an east-southeast direction. It flows into the Mediterranean Sea, forming a delta in the Province of Tarragona, in southern Catalonia. In the Iberian peninsula, it ranks second in length after the Tagus and second in discharge volume, and drainage basin, after the Douro. It is the longest river entirely within Spain; the other two mentioned flow into Portugal. It is also the second-longest river in the Mediterranean basin, after the Nile.