In August 1936 Harry Pollitt arranged for Tom Wintringham to go to Spain to represent the CPGB during the Civil War. While in Barcelona he developed the idea of a volunteer international legion to fight on the side of the Republican Army. He wrote: "You have to treat the building of an army as a political problem, a question of propaganda, of ideas soaking in."
On 10th September 1936 Wintringham wrote to Harry Pollitt that he had arranged for Nat Cohen, a Jewish clothing worker from Stepney, to establish "a Tom Mann centuria which will include 10 or 12 English and can accommodate as many likely lads as you can send out... I believe that full political value can only be got from it (and that's a lot) if its English contingent becomes stronger. 50 is not too many."
Maurice Thorez, the French Communist Party leader, also had the idea of an international force of volunteers to fight for the Republic. Joseph Stalin agreed and in September 1936 the Comintern began organising the formation of International Brigades. An internatinal recruiting centre was set up in Paris and a training base at Albacete in Spain.
Battalions estblished included the Abraham Lincoln Battalion, British Battalion, Connolly Column, Dajakovich Battalion, Dimitrov Battalion, Mackenzie-Papineau Battalion, George Washington Battalion, Mickiewicz Battalion and Thaelmann Battalion.
A total of 59,380 volunteers from fifty-five countries served during the Spanish Civil War. This included the following: French (10,000), German (5,000), Polish (5,000), Italian (3,350), American (2,800), British (2,000), Canadian (1,000), Yugoslavian (1,500), Czech (1,500), Canadian (1,000), Hungarian (1,000) and Scandinavian (1,000). These men were organized into the 11th, 12th, 13th, 14th and 15th of the Mixed Brigades.
Men who fought with the Republican Army included George Orwell, André Marty, Christopher Caudwell, Jack Jones, Len Crome, Oliver Law, Tom Winteringham, Joe Garber, Lou Kenton, Bill Alexander, David Marshall, Alfred Sherman, William Aalto, Hans Amlie, Bill Bailey, Robert Merriman, Fred Copeman, Tom Murray, Steve Nelson, Walter Grant, Alvah Bessie, Joe Dallet, David Doran, John Gates, Harry Haywood, Oliver Law, Edwin Rolfe, Milton Wolff, Hans Beimler, Frank Ryan, Emilo Kléber, Ludwig Renn, Gustav Regler, Ralph Fox, Sam Wild and John Cornford.
Women were active supporters of the International Brigades. A large number of women volunteered to serve in Medical Units in Spain during the war. This included Annie Murray, Thora Silverthorne, Salaria Kea, Mildred Rackley, Sylvia Townsend Warner, Mary Valentine Ackland, Lillian Urmston and Penny Phelps.
Volunteers came from a variety of left-wing groups but the brigades were always led by Communists. This created problems with other Republican groups such as the Workers Party of Marxist Unification (POUM) and the Anarchists. One of the NKVD agents in Spain, Walter Krivitsky, claimed: ""Stalin's intervention in Spain had one primary aim - and this was common knowledge among us who served him - namely, to include Spain in the sphere of the Kremlin's influence... The world believed that Stalin's actions were in some way connected with world revolution. But this is not true. The problem of world revolution had long before that ceased to be real to Stalin... He was also moved however, by the need of some answer to the foreign friends of the Soviet Union who would be disaffected by the great purge. His failure to defend the Spanish Republic, combined with the shock of the great purge, might have lost him their support."
Alexander Orlov and his NKVD agents had the unofficial task of eliminating the supporters of Leon Trotsky fighting for the Republican Army and the International Brigades. This included the arrest and execution of leaders of the Worker's Party (POUM), National Confederation of Trabajo (CNT) and the Federación Anarquista Ibérica (FAI). Edvard Radzinsky, the author of Stalin (1996) has pointed out: "Stalin had a secret and extremely important aim in Spain: to eliminate the supporters of Trotsky who had gathered from all over the world to fight for the Spanish revolution. NKVD men, and Comintern agents loyal to Stalin, accused the Trotskyists of espionage and ruthlessly executed them." Orlov later claimed that "the decision to perform an execution abroad, a rather risky affair, was up to Stalin personally. If he ordered it, a so-called mobile brigade was dispatched to carry it out. It was too dangerous to operate through local agents who might deviate later and start to talk."
According to the authors of Deadly Illusions (1993) in March 1937 General Yan Berzin had sent a confidential report to War Commissar Kliment Voroshilov "reporting resentment and protests he had received about the NKVD's repressive operations from high Republican officials. It stated that the NKVD agents were compromising Soviet authority by their excessive interference and espionage in Government quarters. They were treating Spain like a colony. The ranking Red Army General concluded his report with a demand that Orlov be recalled from Spain at once." Abram Slutsky, the head of the Foreign Department of NKVD, told Krivitsky. "Berzin is absolutely right our men were behaving in Spain as if they were in a colony, treating even Spanish leaders as colonists handle natives".
Krivitsky admitted: " Already in December 1936, the terror was sweeping Madrid, Barcelona and Valencia. The OGPU had its own special prisons. Its units carried out assassinations and kidnappings. It filled hidden dungeons and made flying raids. It functioned, of course, independent of the Loyalist government. The Ministry of Justice had no authority over the OGPU, which was an empire within an empire. It was a power before which even some of the highest officers in the Caballero government trembled. The Soviet Union seemed to have a grip on Loyalist Spain, as if it were already a Soviet possession."
Antony Beevor, the author of The Spanish Civil War (1982), has argued: "The persistent trouble in the Brigades also stemmed from the fact that the volunteers, to whom no length of service had ever been mentioned, assumed that they were free to leave after a certain time. Their passports had been taken away on enlistment. Krivitsky claimed that these were sent to Moscow by diplomatic bag for use by NKVD agents abroad. Brigade leaders who became so alarmed by the stories of unrest filtering home imposed increasingly stringent measures of discipline. Letters were censored and anyone who criticized the competence of the Party leadership faced prison camps, or even firing squads. Leave was often cancelled, and some volunteers who, without authorization, took a few of the days owing to them, were shot for desertion when they returned to their unit. The feeling of being trapped by an organization with which they had lost sympathy made a few volunteers even cross the lines to the Nationalists. Others tried such unoriginal devices as putting a bullet through their own foot when cleaning a rifle (10 volunteers were executed for self-inflicted wounds)."
Walter Krivitsky, confirmed the story about the use of passports: "Several times while I was in Moscow in the spring of 1937, I saw this mail in the offices of the Foreign Division of the OGPU. One day a batch of about a hundred passports arrived, half of them American. They belonged to dead soldiers. That was a great haul, a cause for celebration. The passports of the dead, after some weeks of inquiry into the family histories of their original owners, are easily adapted to their new bearers, the OGPU agents."
The International Brigades played an important role in the defence of Madrid in November 1936. They also suffered heavy losses at Jarama (February 1937), Brunete (July, 1937), Teruel (December 1937) and Ebro (July-August 1938). On 25th September 1938, Juan Negrin, head of the Republican government, announced for diplomatic reasons that the International Brigades would be unilaterally withdrawn from Spain. However, General Francisco Franco failed to reciprocate and German and Italian forces remained to continue the struggle.
John Gates recalled in The Story of an American Communist (1959): "The main farewell took place in Barcelona on Oct. 29. For the last time in full uniform, the International Brigades marched through the streets of Barcelona. Despite the danger of air raids, the entire city turned out... We paraded ankle-deep in flowers. Women rushed into our lines to kiss us. Men shook our hands and embraced us. Children rode on our shoulders. The people of the city poured out their hearts. Our blood had been shed with theirs. Our dead slept with their dead. We had proved again that all men are brothers."
Before leaving for home Sam Wild, commander of the British Battalion, was quoted as saying: "The British Battalion is prepared to carry on the work begun here to see to it that our 500 comrades who sleep for ever beneath Spanish soil shall serve as an example to the entire British people in the struggle against fascism."
In October 1938, the 12,673 members of the International Brigades still left in Spain began to leave the country.
It is estimated that 59,380 foreign volunteers fought in the Spanish Civil War. Of these, 9,934 (16 per cent) died and 7,686 (12.9 per cent) were badly wounded.