The Battle of the Ebro (Spanish: Batalla del Ebro, Catalan: Batalla de l'Ebre) 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 a massive blow to the Second Spanish Republic, with tens of thousands left dead or wounded and little effect on the advance of the Nationalists.

By 1938, the Second Spanish Republic was in dire straits. The Republican Northern zone had fallen, and in the winter of 1937/38 the Republican Popular Army had spent its forces in the Battle of Teruel, a series of bloody combats in subzero temperatures around the city of Teruel, which was retaken by the Francoist army in February.

Then, the Nationalists launched an offensive in Aragon in March without giving their enemies a chance to recover. Fighting in the middle of bitter winter temperatures, the exhausted Republican army could offer only feeble resistance. By 15 April Franco's troops reached the Mediterranean Sea at Vinaròs, cutting Republican territory in two. As a result, the Nationalist army conquered Lleida and the hydroelectric dams that provided much of the Catalan industrial areas with electricity.

Nevertheless, on 17 March 1938, after the Anschluss, the French government decided to reopen the frontier. The Republican Army in Catalonia received 18,000 tons of war material between March and mid-June and twelve new divisions were formed from Nationalist prisoners-of-war and an extended call-up, which included conscripts that ranged in age from sixteen years old, the so-called Quinta del Biberón (the baby-bottle call-up), to middle-aged fathers. A new army, the Ebro's army, was then formed.

Meanwhile, the Francoist armies attacked the XYZ Line north of Valencia with the intention of capturing the Republican capital, instead of advancing towards Barcelona, fearing that France would enter the war in support of the ailing Republic. In response to the situation, Spanish premier Juan Negrín approved a plan by General Vicente Rojo Lluch to launch attacks against the main Francoist forces advancing towards Valencia. The purpose of the attacks was to relieve the pressure on Valencia and Catalonia, as well as to show European governments that the Republican government was still viable.

In order to distract the Nationalist armies that were advancing towards Valencia, the Popular Republican Army decided upon an offensive in the lower Ebro basin. The army was large, but it lacked enough air and artillery support. The Ebro Army was formed on 15 May under Lieutenant Colonel Juan Modesto, merging the 15th and the 5th Army Corps. It would receive reinforcements from the 12th and 18th Army Corps as soon as the battle began.

The 15th Army Corps XV Cuerpo del Ejército was led by Manuel Tagüeña from Escaladei and was formed by the following divisions:

The 5th Army Corps V Cuerpo del Ejército Popular, led by Lieutenant Colonel Enrique Líster, with base in Salou: