Complex sentence: The Army of the Ebro was formed on May 15th under Lieutenant Colonel Juan Modesto, merging the 15th and the 5th Army Corps, that would receive reinforcement from the 12th and 18th Army Corps as soon as the battle began.
Split into simpler sentences: The Army of the Ebro was formed on May 15th under Lieutenant Colonel Juan Modesto, merging the 15th and the 5th Army Corps. It would receive reinforcement from the 12th and 18th Army Corps as soon as the battle began.