Juan Modesto was born in Spain in 1906. He joined the Spanish Army and served in Morocco. A member of the Communist Party (PCE), in 1933 he was appointed commander of the Milicias Antifascistas Obreras y Campesinas (MAOC).
On the outbreak of the Spanish Civil War the MAOC was integrated into the Fifth Army Corps which played an important role in the defence of Madrid in 1936.
In October 1936 Modesto was appointed commander of the Fifth Army Corps and took part in the battle of Jarama in February 1937. This outstanding soldier was involved in most of the major offensives over the next two years including Ebro (July-August 1938).
In March 1938 Juan Negrin promoted Modesto to the rank of general and gave him command of the Army of Ebro. He also promoted other communist leaders such as Antonio Cordon and Enrique Lister to senior posts in the army. Segismundo Casado, commander of the Republican Army of the Centre, now became convinced that Negrin was planning a communist coup.
On 4th March, Casedo, with the support of the socialist leader, Julián Besteiro and disillusioned anarchist leaders, established an anti-Negrin National Defence Junta. On 6th March José Miaja in Madrid joined the rebellion by ordering the arrests of Communists in the city.
Modesto fled to Latin America but later settled in Czechoslovakia. In 1969 he published his autobiography, I am of the Fifth Regiment.
Juan Modesto died in Prague in 1969.