As time passes and the bloodletting and desperation of the war increases, the situation in the embassies harbouring refugees in Republican Madrid gets more and more precarious, and the fear of attacks intensifies, so that anyone who has a reasonable possibility of escape prefers to run the risk of the adventure in search of a safe refuge rather than prolong the anxious uncertainty of confinement and waiting. That's what Samuel Ros does, arriving in Chile in the middle of 1937, not to return to Nationalist Spain until the following year. Encouraged by Ros's success, at some point in the autumn of 1937 Sánchez Mazas attempts to escape. He has the help of a prostitute and a young Falangist sympathizer whose family has or had a transport company, and who are acquaintances of Sánchez Mazas. His plan is to get to Barcelona and, once there, engage the help of the fifth column to make contact with the escape networks that smuggled people across the French border. They put the plan into action and, for several days, Sánchez Mazas travels by secondary roads and cart tracks, camouflaged under a load of rotting vegetables, the 600 kilometres between him and Barcelona in the company of the prostitute and the young Falangist. Miraculously, they make it past all the control posts and arrive safe and sound at their destination, with no setbacks more serious than a blown-out tyre and getting the fright of their lives from a dog with an overly sensitive nose. The three travellers separate in Barcelona, and Sánchez Mazas is received, just as planned, by a lawyer who belongs to the JMB, one of the numerous, unconnected Falangist factions the fifth column have scattered throughout the city. After granting him a few days rest, the members of the JMB urge him to take command and, asserting his right as member number four of the Falange, assemble all the fifth columnist splinter groups and submit them to party discipline, obliging them to coordinate their activities. Perhaps because his only preoccupation up to that moment has been to get out of the red zone and cross over into Nationalist territory, or simply because he knows himself incapable of action, the offer surprises him, and he refuses outright on the grounds of his complete ignorance of the situation in the city and the groups operating within it; but the members of the JMB, who are as young and bold as they are inexperienced, and who greeted his arrival as a gift from providence, insist, and he has no choice but to accept.
Over the following days Sánchez Mazas meets with representatives of other fifth-column factions and one morning, on his way to the Iberia, a bar in the city centre where the owner is a sympathizer of the Nationalist cause, he is arrested by military intelligence agents. This is 29 November 1937; versions of what happens next differ. There are those who maintain that Father Isidoro Martin, who had been Sánchez Mazas' professor at the Maria Cristina Royal College at El Escorial, intercedes in vain on his behalf, to Manuel Azana, who had also been a student of his at that school. Julián de Zugazagoitia, whom at the end of the war Sánchez Mazas unsuccessfully tried to save from the firing squad, affirms that he proposed to President Negrín that they exchange him for the journalist Federico Angulo, and that Azana hinted at the expediency of swapping the writer for some compromising manuscripts of his own that were in seditious hands. Another version maintains that Sánchez Mazas didn't even make it as far as Barcelona, because after leaving the Chilean Embassy he sought refuge in the Polish one, which was attacked, and that was when Azorín intervened to spare him a death sentence. There are even those who claim Sánchez Mazas actually was swapped at some point during the war. These last two hypotheses are erroneous; almost certainly the first two are not. However it happened, the truth is that, after being arrested by the SIM, Sánchez Mazas was sent to the Uruguay, a ship anchored in the port of Barcelona and converted into a floating prison earlier in the war, and later taken to the Palace of Justice, where he was tried along with other fifth columnists. During the trial he was accused of being the Commander-in-Chief of the fifth column in Barcelona, which was false, and of incitement to rebellion, which was true. However, unlike most of the rest of the accused, Sánchez Mazas was not condemned to death. This is puzzling; perhaps only another intervention from afar by Indalecio Prieto can explain it.
Once the trial is over, Sánchez Mazas is returned to the Uruguay, and passes the following months in one of its cells. The living conditions are not good: food is scarce; the treatment brutal. News of the war is also scarce, but as it progresses, even the prisoners on the Uruguay understand that Franco's victory is near. On 24 January 1939, two days before Yagüe's troops enter Barcelona, he's awakened by an unusual sound, and before long notices the jailers' nervousness. For a moment he thinks they're going to release him; then he thinks they're going to shoot him. He spends the morning lurching between these two agonizing alternatives. At about three in the afternoon a SIM agent orders him out of his cell, off the boat and onto a bus parked on the dock, where another fourteen prisoners from the Uruguay and the Vallmajor checa are waiting, along with the sixteen SIM agents in charge of their custody. Among the prisoners are two women, Sabina González de Carranceja and Juana Aparicio Pérez del Pulgar; also among them is José Maria Poblador, an early leader of the JONS and important player in the putsch of July 1936, and Jesus Pascual Aguilar, one of the leaders of the Barcelona fifth column. What no one can know at this moment is that of all the male prisoners making up the convoy, by the end of the week only Sánchez Mazas, Pascual and Poblador will still be alive. Silently the bus crosses Barcelona, which has been changed by the terror of exodus and the wintry sky into a ghostly desolation of boarded-up windows and balconies, and wide ashen avenues with the disorderly air of an abandoned refugee camp, and traversed only, if at all, by furtive transients who gnash their teeth like wolves looking hungry and ready to flee as they pass craters in the pavement, protected from adversity and from the glacial wind only by threadbare overcoats. Upon leaving Barcelona by the road to exile, the spectacle turns apocalyptic: an avalanche of men and women, old people and children, soldiers and civilians together, carrying clothing, mattresses and household goods, advancing laboriously with the unmistakable trudge of the defeated or riding on carts or mules of despair, the road and ditches overflowing with people strewn intermittently with corpses of animals with their guts exposed or abandoned vehicles. The caravan crawls forward interminably. Sometimes it stops; sometimes, with a mixture of disbelief, hatred and immeasurable weariness, someone stares hard at the occupants of the bus, envying their comfort and shelter, ignorant of their firing squad fate; every now and then someone hurls an insult. Sometimes, as well, a Nationalist airplane flies over the road and spits out a few bursts of machine-gun fire or drops a bomb, provoking a panicked stampede among the refugees and a faint hope among the prisoners on the bus, who at some point even cherish the illusion — soon belied by the strict watch the SIM agents keep over them — that they could take advantage of the chaos of an attack to escape across the countryside.