The council seized power with Colonel Segismundo Casado's coup on 5 March 1939 when it was clear that the Republicans had lost the war.

The leaders hoped to negotiate an end to hostilities with the rebel forces led by General Francisco Franco.

However, Franco insisted on unconditional surrender, and on 26 March 1939 launched the final offensive of the Spanish Civil War.

[1] With the news of the fall of Barcelona on 26 January 1939, and Azaña's resignation as president, Besteiro decided to seek peace.

[2] Besteiro wanted to dissolve the Popular Front and replace it with a government that excluded communists, since he thought the policy of the Western democracies towards the civil war was determined by anti-communism rather than appeasement of Hitler and Mussolini.

[2] The Federación Anarquista Ibérica (FAI, Iberian Anarchist Federation) tried to persuade President Manuel Azaña to dismiss the government of Juan Negrín at the start of December 1938, before the rebel Catalonia Offensive.

General José Miaja, who had supreme command of the Republican forces, communicated with Negrin, who was in Toulouse, on 9 February 1939.

[9] When President Negrín refused to surrender to Francisco Franco, Mera decided to support Casado and Besteiro in their coup.

[15] Mera reached the Ministry of Finance at 9 p.m.[16] The formation of the National Defense Council (Consejo Nacional de Defensa) was announced on Union Radio at 10 p.m.

But with Miaja as president, many professional officers in Madrid, New Castile, the Levante and the remainder of Republican-held territory accepted the authority of the council.

He and his former Foreign Secretary Julio Álvarez del Vayo boarded a plane for France late in the evening of 6 March 1939.

[19] The controversial decision of the anarchist CNT-FAI to collaborate with Casado and his political supporters in the council proved vital to the success of the coup.

[28] To prevent pointless waste of life the council let Republican troops disband, and arranged to evacuate civilians and soldiers who wanted to leave Madrid.

[29] On 12 March 1939, Casado communicated to the rebel government that he and General Manuel Matallana wanted to come to Burgos to negotiate peace terms.

They requested a political amnesty, time to be given for those who wanted to leave Spain to do so, and "respect for the lives, liberties, and careers of professional soldiers.

[33] The council abolished the Servicio de Investigacion Militar (SIM) intelligence agency, which seems to have mostly been staffed by PSOE or UGT members, a few days before the rebels occupied Madrid.

[35] A second meeting was held at the Gamonal aerodrome on 25 March 1939, where the Republican Lieutenant Colonel Antonio Garijo was told the pace of surrender was too slow.

[36] When the National Defense Council heard the report of this meeting they gave orders for immediate delivery of the Republican aviation.

[35] That day the council left for Valencia, and Casado told his troops to disarm and offer no resistance to the rebels.

[35] Julián Besteiro and Rafael Sánchez Guerra were the only two members of the council who remained in the capital when rebel troops entered the city.

[40] At dawn on 30 March 1939, Casado, with 143 men, 19 women and 2 children, boarded the British ship HMS Galatea at Gandia.

[42][43] Carrillo stayed in Britain during the war, where he continued to attack Negrín, then moved to France and was a member of the executive committee of the PSOE.