On 17 October 1936, an open letter by Joseph Stalin to José Díaz was published in Mundo Obrero, arguing that victory for the Spanish second republic was a matter not only for Spaniards but also for the whole of "progressive humanity"; in short order, communist activists joined with moderate socialist and liberal groups to form anti-fascist "popular front" militias in several countries, most of them under the control of or influenced by the Comintern.

Finally, some 500 communists who had been exiled to Russia were sent to Spain (among them, experienced military leaders from the First World War like "Kléber" Stern, "Gomez" Zaisser, "Lukacs" Zalka and "Gal" Galicz, who would prove invaluable in combat).

Keith Scott Watson, a journalist who fought alongside Esmond Romilly at Cerro de los Ángeles and who later "resigned" from the Thälmann Battalion, describes in his memoirs how he was detained and interrogated by Anarchist border guards before eventually being allowed into the country.

It was run by a troika of Comintern heavyweights: André Marty was commander; Luigi Longo (Gallo) was Inspector-General; and Giuseppe Di Vittorio (Nicoletti) was chief political commissar.

At the end of the fight, the Nationalist troops had been forced to retreat, abandoning all hopes of a direct assault on Madrid by Casa de Campo, while the XIth Brigade had lost a third of its personnel.

As a result of language and communication problems, command issues, lack of rest, poor coordination with armored units, and insufficient artillery support, the attack failed.

On 19 November, the anarchist militias were forced to retreat, and Nationalist troops — Moroccans and Spanish Foreign Legionnaires, covered by the Nazi Condor Legion — captured a foothold in the University City.

(It is doubtful that Delasalle would have been a spy for Francisco Franco; he was denounced by his second-in-command, André Heussler, who was subsequently executed for treason during World War II by the French Resistance.)

The whole Italian expeditionary corps — 35,000 men, with 80 battle tanks and 200 field artillery — was deployed, as Benito Mussolini wanted the victory to be credited to Italy.

Existing primary sources provide conflicting information as to the number of brigadiers killed; a report of the IB Albacete staff from late March 1938 claimed 4,575 KIA,[23] an internal Soviet communication to Moscow by an NKVD major Semyon Gendin from late July 1938 claimed 3,615 KIA,[24] while the prime minister Juan Negrín in his farewell address in Barcelona of October 28, 1938, mentioned 5,000 fallen.

[69] Among smaller contingents, the KIA ratio calculated appears to be 10% for the Cubans,[70] 18% for the Austrians,[71] 21% for the Balts (Estonians, Latvians, Lithuanians),[72] 21-25% for the Swiss,[73] 31% for the Finns,[74] 13%-33% for the Greeks,[75] 23-35% for the Swedes,[76] 40% for the Danes,[77] and 44% for the Norwegians.

The disbandment was part of an ill-advised effort to get the Nationalists' foreign backers to withdraw their troops and to persuade the Western democracies such as France and Britain to end their arms embargo on the Republic.

[81] Perhaps half of the International Brigadistas were exiles or refugees from Nazi Germany, Fascist Italy or other countries, such as Hungary, which had authoritarian right-wing governments at the time.

[5] The first brigades were composed mostly of French, Belgian, Italian, and German volunteers, backed by a sizeable contingent of Polish miners from Northern France and Belgium.

[83] After the Civil War was eventually won by the Nationalists, the brigaders were initially on the "wrong side" of history, especially as most of their home countries had right-wing governments (in France, for instance, the Popular Front was not in power anymore).

Two became prime ministers: Mehmet Shehu (Albania, 1954–1981) and Ferenc Münnich (Hungary, 1958–1961), while Heinrich Rau was the chairman of DWK, sort of government of what would become East Germany (1948–1949).

In communist countries tens of ex-combatants served as ministers (e.g. Karlo Lukanov in Bulgaria, Josef Pavel in Czechoslovakia, Gheorghe Vasilichi in Romania, Ernő Gerő in Hungary, Maks Baće in Yugoslavia), or held other key state jobs, especially in the army and security (e.g. Erich Mielke in East Germany).

[120] In 2001, the few surviving Canadian veterans of the Spanish Civil War dedicated a monument in Ottawa's Green Island Park to their country's International Brigaders.

[121] Though after 1945, they were celebrated in all communist states as freedom fighters against fascism, their position was secondary and the official narrative centred upon other threads, e.g. the USSR-raised army in Poland, the Slovak National Uprising in Czechoslovakia, or the partisan quasi-state in Yugoslavia.

No such narrative was available in the case of East Germany, whose “communist government found itself without historical roots beyond the Soviet occupation of Eastern Europe and turned the heroism of the Spanish Civil War fighters into the myth that became a central focus of the German Democratic Republic”.

They took three of the most important military posts: Heinz Hoffmann as commander of Nationale Volksarmee, Erich Mielke as head of Ministry for Security, and Friedrich Dickel as Minister of Interior.

There were 10 former interbrigadistas who entered the Political Bureau of Sozialistische Einheitspartei Deutschlands, some briefly (e.g. Anton Ackermann, Dahlem or Zaisser) and some for decades (e.g. Paul Verner, Kurt Hager and Alfred Neumann).

After 1989 there was some confusion in both Czechia and Slovakia as to how the IB veterans should be approached, though the controversy was not comparable to the similar one in Poland;[141] eventually the image which seems to prevail is this of anti-fascist combatants.

[143] In line with the 1920 legislation, Polish citizens who volunteered to the IB were automatically stripped of citizenship as individuals who without formal approval served in foreign armed forces.

[156] Though from the onset Polish engagement in IB was hailed as "working class taking to arms against Fascism", the most intense idolization took place between the mid-1950s and the mid-1960s, with a spate of publications, schools and streets named after Dąbrowszczacy.

[160] Unlike in East Germany, except Szyr no-one made it to the very top strata of the Communist elite (member of Political Bureau of PZPR, minister).

A state-run institution IPN declared Polish IB combatants in service of the Stalinist regime and related homage references subject to de-communisation legislation.

[165] In Switzerland, public sympathy was high for the Republican cause, but the federal government banned all fundraising and recruiting activities a month after the start of the war as part of the country's long-standing policy of neutrality.

[172] They arrived at Victoria Station in central London on 7 December and were met warmly as returning heroes by a crowd of supporters including Clement Attlee, Stafford Cripps, Willie Gallacher, Ellen Wilkinson and Will Lawther.

[175] In the United States, the returned volunteers were labeled "premature anti-fascists" by the FBI, denied promotion during service in the U.S. military during World War II, and pursued by Congressional committees during the Red Scare of 1947–1957.