Agostini left the United States for Spain in 1937 to fight in defense of the Spanish Republic against the rise of Francisco Franco's forces.

[2] In a morale contribution during the war, the Cuban brigade had its own battle hymn that was titled "Combatientes de la libertad" ("Freedom fighters"); set to music by popular Cuban composer Julio Cuevas Díaz, with its lyrics rewritten by Agostini (then Secretary of the Contingent), to read "We are the volunteers, the freedom fighters / black and white, of all races / united, united more and more / for democracy and universal peace.

"[2] However, as the war turned in favor of the Falangists, the Republican forces tasked Agostini, now with the rank of colonel, as among the chiefs responsible for the transfer of the International Brigades to Barcelona and ultimately out of Spain later that year.

Upon reaching France through the Pyrenees with other ex-combatants as well as refugees, Agostini was detained by French forces in camps for processing for an extended period before finally being released to Cuba in 1940.

[4] Aboard the Cinco Hermanas he was transporting a shipment of explosives when he received urgent instructions to anchor in Cárdenas Bay and go to the Presidential Palace, where he was informed that President Grau decided to suspend the project's execution.

[2] As his governmental duties reduced during this period from the end of Grau's term and into Prío's, Agostini turned his focus back to international competitive athletics, especially fencing.

He also was part of the National Team in the World Shooting Championship in Los Angeles, California, in September 1951, and represented Cuba in several competitions in North America, including Mexico, Panama, Puerto Rico and mainland United States.

[2] On 9 March 1952 he arrived in Key West, Florida, with his wife, Enma Surí Ramírez, another accomplished athlete, for a pistol shooting competition.

He covertly established contacts with soldiers and officers who had been discharged from the armed forces (navy, army) and police by Batista's allies and accomplices.

[2] Agostini's established international reputation and prestige led him to be accepted by many leading figures from other organizations, mainly from the Orthodox Party.

These prominent figures and supporters included: Pelayo Cuervo Navarro and the prominent psychiatrist F. René de la Huerta in Havana, José Manuel Gutiérrez in Matanzas, the former Orthodox Party candidate for Mayor of Santa Clara Santiago Riera, as well as the Orthodox leader of Marianao, Juan Manuel Márquez.

In May 1955 he had continued to be in hiding and conducting covert activities, now waiting to hear from former President Carlos Prío and what the other exiled hierarchy of insurrectionary Authentic Party would leadership decide to do.

[2] On 9 June 1955, Agostini was at the home of Dr. René de la Huerta, a leading member of the influential Agrupación Católica Universitaria, in Vedado holding a secret meeting with other insurrection leaders Menelao Mora, Lomberto Díaz, and the Orthodox Party jurist Pelayo Cuervo Navarro.

[2] Rising revolutionary leader Fidel Castro, then a young lawyer who was recently released from incarceration, seized on the event to make the case against Batista's regime to the public.

"[9] Other anti-Batista figures would also speak out in the aftermath of Agostini's death, including José Antonio Echeverría and René Anillo, both leaders of the Directorio Revolucionario Estudiantil (DRE).

[2] Agostini's widow, Emma Surí, initiated a legal case where she presented a private accusation to the Emergency Court of Havana.

[2] Manuel Antonio Varona Loredo, President of the Authentic Party, assumed the representation of Agostini's widow and acted as prosecutor in the summary.