As the leader of the moderate wing of the Spanish Socialist Workers' Party (PSOE), he unsuccessfully tried to reach an agreement with the nationalist faction during the last year of the Civil War.

Soon after, Besteiro became a member of Unión General de Trabajadores (UGT) trade union, and of the Spanish Socialist Workers' Party (PSOE).

Opinion within the PSOE turned against Besteiro as the Primo de Rivera regime became more unpopular during the economic downturn as part of the Great Depression.

After the Spanish Civil War broke out in 1936, and against the urgings of friends, he refused to leave Madrid and seek exile.

[2] Manuel Azaña, the Spanish Republican president, chose Besteiro as the representative to the coronation of the new British King George VI in London on 12 May 1937.

There he met with Sir Anthony Eden to seek British aid in ending the war, which had attracted German and Russian support of the opposing sides, but no significant results followed.

[9] After the failure of his mission in London, Besteiro returned to work on the town council of Madrid but withdrew from other official public life.

With the news of the fall of Barcelona on 26 January 1939 and Azaña's resignation as President of the Republic, Besteiro decided to work to achieve peace and stop the resistance.

On 5 March Besteiro announced the creation of the National Defence Council (Consejo Nacional de Defensa).