Later in the Civil War, the unit operated in Catalonia, defending the Republican defensive line along the Segre river.

After the May Days battles in Barcelona, the Republican government recovered its competencies in the area of defence, and General Sebastián Pozas assumed command of military forces in Catalonia.

In just a few weeks the Eastern Army was practically destroyed by the attacking "nacionales",[12] additionally losing a great amount of territory.

As a consequence, General Pozas was removed from his post, and in his place the colonel Juan Perea Capulino was installed.

Towards the end of 1938, during the first few days of the Catalonia Offensive, the Eastern Army maintained a bitter resistance against the advancing enemy.