He moved to Seville, where he had contact with José Díaz Ramos, who would become general secretary of the Spanish Communist Party (PCE: Partido Comunista Español).

[3][5] The historian Gregorio Morán says that when the Comintern ordered the appointment of Checa as secretary of the PCE, he was not even a substitute member of Central Committee.

[6][7] During the Spanish Civil War (17 July 1936 – 1 April 1939) Checa and "Pasionaria" (Dolores Ibárruri) led the PCE due to illness of the Secretary General, José Díaz.

This consisted of three ministries, War, Supplies and Public Order, which were led by Antonio Mije, Pablo Yagüe and Santiago Carrillo, respectively.

[9] As secretary of the organization, he was also responsible for the secret or illegal party apparatus, consisting of "special teams" that, for example, had already taken part in murdering Falangists.

"[10] Julius Ruiz does not directly implicate Checa and argues that the killings were mainly managed and promoted by the Spanish, with the Soviets simply giving support.

[14] When the Republican forces were defeated in the Battle of the Ebro (July–November 1938) and General Francisco Franco's troops reached the Mediterranean, Checa stayed in Madrid with Luis Cabo Giorla, Jesús Hernández Tomás, Isidoro Diéguez Dueñas and Antonio Mije while the main Communist leadership center was established in Barcelona.

[15] During the Catalonia Offensive Checa, Dolores Ibárruri and Manuel Delicado formed a three-member secretariat that took the leadership of the party in the absence of José Díaz and installed itself near the government.

[18] On 4 March 1939 there was a counter-insurrection in the Cartagena naval base that quickly became a pro-Nationalist uprising, but was reduced in two days by the 206th Mixed Brigade commanded by Artemio Precioso Ugarte.

On 5 March 1939 Checa, Fernando Claudín, Palmiro Togliatti and other Communist leaders moved to the base of the 206th Mixed Brigade to insist on the need to suppress the rebellion.

Only Checa stayed in Spain, with Fernando Claudín and Palmiro Togliatti, to evacuate all the communist cadres they could find and maintain the clandestine structure of the PCE.

They were released after appealing to the local commander of the Servicio de Información Militar (secret service), Prudencio Sayagués, an old friend of Claudin from their days in the Federación Universitaria Escolar (FUE).

[27] Both the Comintern and the Central Committee of the PCE blamed Checa, Togliatti, Uribe and Hernández for failing to defeat the Casado coup.

[31] He was there when the pact was signed between Nazi Germany and the Soviet Union in August 1939, which led to the banning of the French and Spanish Communist parties.

According to a confidential report from another communist militant to the PCE leadership in Moscow, writing about Checa and the murder of Trotsky, "the party [in Mexico] has been informed that the [Mexican] police has a file containing detailed records of the activity of the Spanish comrades.