Not even when the Republic was fighting for its life, in the last, gargantuan battle at the Ebro between July and November 1938, did the immense pressures of the war override constitutional guarantees. In October 1938, the POUM leaders were brought to trial and convicted of having publicly supported an illegal rebellion (that of May 1937 in Barcelona) against the Republican state at war. That they were brought before the courts at this late stage was certainly about making an example of them in order to discipline the home front at a time when its disintegration threatened. But the POUM’s was not a show trial. In spite of the Spanish Communist Party leadership’s best efforts to influence proceedings, and thus to cashier its rival – including by a menacing publicity campaign – the trial observed due constitutional process. The Republic’s political culture remained democratic against all the odds.

The Ebro offensive, without which the POUM trial cannot properly be understood, was also the Republic’s last throw of the dice. It had three objectives: to protect Valencia from Francoist conquest (first attempted by Italian forces); to restore contact with Catalonia, thus reuniting the two Republican zones; and thirdly to demonstrate to an international audience the resilience of the Republican army and its capacity to plan and implement offensive action. British Foreign Office opinion noted at the end of 1938 that ‘the Ebro campaign was without any doubt a great [Republican] Government victory’. It also noted that Franco was more heavily reliant than ever on Italy and Germany. This was especially true in terms of airpower. The Ebro saw massive air battles that were unprecedented in the history of warfare and would not be seen again until the Battle of Britain in the opening stages of the Second World War.

Franco paid Nazi Germany in mining rights to guarantee his air superiority at the Ebro. Whatever his assurances to British diplomats in 1936 that this would not happen, by 1938 Franco’s desperate need for more airpower to win the war meant he was prepared to cede what he had previously resisted. The product of these valuable mineral concessions played a vital part in Germany’s re-armament programme. But the military advantage Franco gained paid immense dividends in the short term. Republican communications were bombed to oblivion and, as so many international brigader memoirs testify, their troops were blasted off the bare and rocky hillsides by the sheer force of the incendiary materiel launched.

In military terms, all the intervening powers were keen to use the opportunity provided by the Spanish war to train their personnel and to test equipment and strategies in real and extended combat conditions – although these were collateral benefits rather than motives for their initial intervention. Germany and Russia welcomed the chance to try out new technology – above all against the other, understood as the eventual ‘territorial’ opponent. So it was that in Spain elements of what would become Blitzkrieg made their appearance, while the Soviet Union benefited especially from being able to test its tanks and armoured cars. But it was the war in the air – in which nearly 3,000 planes participated – that really marked Spain out in terms of technical and technological innovation (for example, the precision bombing of specific targets or new techniques for dealing with anti-aircraft fire).

In the end, in November 1938, the Republican forces had to retreat back across the River Ebro, which they had crossed with such feats of engineering ingenuity, improvisation, and tenacity the previous July. The usual problems obtained due to shortages of materiel and reserves. But at the Ebro there was a huge difference. This time the retreat was a function not of military defeat (the Republic had successfully blocked Franco’s attack on Valencia), but of an absolutely devastating political defeat that occurred many miles from Spain.