Largo Caballero became head of the UGT and controlled its newspaper, Claridad. In this position he called for the radicalization of the PSOE. This included "the conquest of political power by the working class by whatever means possible" and the "dictatorship of the proletariat organized as a working-class democracy".
In the summer of 1917 Largo Caballero became involved in the organization of a political strike in Spain. The strikers demanded the establishment of a provisional republican government, elections to a constituent Cortes and action to deal with inflation. In Madrid members of the strike committee, including Largo Caballero and Julián Besteiro, were arrested and sentenced to life imprisonment. However, he was released the following year.
Largo Caballero's views were attacked by Indalecio Prieto, the leader of the right-wing of the Socialist Party. He wrote that "Largo Caballero is a fool who wants to appear clever. He is a frigid bureaucrat who plays the role of a mad fanatic". Largo Caballero replied that Prieto was "envious, arrogant, and distainful" and was not a socialist "either in his ideas or in his action."
In 1925 Francisco Largo Caballero became leader of the party. He called for "the conquest of political power by the working class by whatever means possible" and the "dictatorship of the proletariat organized as a working-class democracy".
The two men continued to argue throughout the 1920s. Largo Caballero had the support of union members whereas Prieto gained most of his following from the middle class and the intellectuals in the party. By 1930 the PSOE had 20,000 members.
In August 1930 Indalecio Prieto was a central figure in the formation of the Republican coalition known as the Pact of San Sebastián. Julián Besteiro was opposed to the idea but Largo Caballero, gave it his support as he felt it was the only way the Socialist Party would gain power. At a conference held in July 1930, delegates voted by 10,607 to 8,326 to approve the PSOE taking part in a future coalition government.
After Alfonso XIII abdicated in April 1931 both Largo Caballero and Indalecio Prieto joined the new coalition government led by Niceto Alcala Zamora. Largo Caballero served as minister of labour and formulated agrarian policies which called for the distribution of land to landless labourers. This increased the support for the PSOE in rural communities. By 1935 the PSOE had increased its membership to 75,000.
Attacked by the extreme left for not being radical enough, the government faced an anarcho-syndicalist uprising at Casas Viejas in January 1933. The government was severely criticized in the Cortes for its approval of the way the Civil Guard and Assault Guard put down the uprising. This included the execution without trial of fourteen prisoners.
In September 1933 the government of Manuel Azaña collapsed and Largo Caballero and other Socialist Party members of the cabinet left office. The following month Indalecio Prieto announced the end of the Republican-Socialist coalition. In the elections that followed in November 1933 the conservative CEDA became the largest party in the Cortes.
On 15th January 1936, Manuel Azaña helped to establish a coalition of parties on the political left to fight the national elections due to take place the following month. This included the Socialist Party, Communist Party (PCE) and the Republican Union Party.
During the early stages of the Spanish Civil War Largo Caballero was critical of the Popular Front government led by José Giral. Even Largo Caballero's opponents agreed that he was a dynamic leader and in September 1936 he was chosen to replace Giral as prime minister. He also took over the important role of war minister.
Largo Caballero brought into his government two left-wing radicals, Angel Galarza (minister of the interior) and Alvarez del Vayo (minister of foreign affairs). He also included four anarchists, Juan Garcia Oliver (Justice), Juan López Sánchez (Commerce), Federica Montseny (Health) and Juan Peiró (Industry) and two right-wing socialists, Juan Negrin (Finance) and Indalecio Prieto (Navy and Air) in his government. Largo Caballero also gave two ministries to the Communist Party (PCE): Jesus Hernández (Education) and Vicente Uribe (Agriculture).
After taking power Largo Caballero concentrated on winning the war and did not pursue his policy of social revolution. In an effort to gain the support of foreign governments, he announced that his administration was "not fighting for socialism but for democracy and constitutional rule."
Largo Caballero introduced changes that upset the left in Spain. This included conscription, the reintroduction of ranks and insignia into the militia, and the abolition of workers' and soldiers' councils. He also established a new police force, the National Republican Guard. He also agreed for Juan Negrin to be given control of the Carabineros.
Largo Caballero resisted pressure from the Communist Party to promote its members to senior posts in the government. He also refused their demands to suppress the Worker's Party (POUM) in May 1937. The Communists now withdrew from the government. In an attempt to maintain a coalition government, President Manuel Azaña sacked Largo Caballero and asked Juan Negrin to form a new cabinet.
At the end off the Spanish Civil War Largo Caballero went to live in France. After the invasion of the German Army he was captured and sent to Dachau Concentration Camp. He survived the Second World War and returned to Paris where he died in 1946.