It had been started in Spain but this was not just about Spain. It was the messenger of the Great War where 17 millions of people would die. It was about all of the world but there are interest led everyone prefer do not intervene.
UK: They prefer to not intervene because;
1 – If they intervene and the civil war expands, then it would mean a war between United Kingdom and Germany, Italy and Japan.
2 – If Franco succeeds, then investment of UK and its capitalists would be secure.
3 – The two main bourgeoisie Spain and UK had good relations and any intervention would make it worse, since Spain main bourgeoisie consists traditional land owners.
4 – The Second Republic was like Bolsheviks and it would be disastrous to have a Bolshevik government on the centre of Europe.
France: The socialists were in power in France and it would be dangerous to be between two Fascist powers if Franco wins. However, France wanted to make some reforms and if they help republicans then it would mean loss of conservative support who had interests and trade relations with Spain land owners. On the other hand, any intervention would mean left the back door open for Germans.
Soviets: Stalin was in power and he did not favour to intervene that war at first. First of all, any revolution similar to Bolsheviks in Europe would be the end of pact against Germany. On the other hand a civil war that takes too long time means loss of time and energy for Germany and this would turn to Soviet like more time to strengthen lines.
However, on the other front;
Germany: Germany had supported the Fascist coup of Franco from the first day. Hitler's aims are a satellite government at the south of France and new opportunities to reach African colonials. At the beginning of the civil war, troops and of Franco led army were spread all over to Spain. They did not have any chance to win a war like that, however Germany's first intervention (cargo planes) let them transport all of their units to the south of Spain and it took only 6 months to reach Madrid.
Italy: The Italian contribution amounted to over 60,000 troops at the height of the war. The involvement helped to increase Mussolini's popularity. Italian military help to Nationalists against the anti-clerical and anti-Catholic atrocities committed by the Republican side worked well in Italian propaganda targeting Catholics.
As a result of non-intervention politics of major powers, republicans, workers, students, teachers and intellectuals of Spain were alone against Franco and his supporters. So Franco reached to Madrid rapidly with the help of Germany and Italy.
But he could not take it, because it had been already said that “They shall not pass”. There was a voluntarily organized militias, who had come together from all over the world. People from all around the world came to fight. Despite lack of media support, people and especially communist international had managed great propaganda during these term. Montages, anthems, songs and posters had played great role. But the slogan “They Shall Not Pass” “No Pasaran” was the main slogan of movement.
This slogan was most famously used during the Battle of Verdun in the First World War by French General Robert Nivelle. However, the speech of Dolores Ibárruri who was a Spanish Republican heroine of the Spanish Civil War and communist politician of Basque origin, have made it unique.
In 19 July 1936 Dolores Ibárruri gave a speech
Workers! Farmers! Anti-fascists! Spanish Patriots! Confronted with the fascist military uprising, all must rise to their feet, to defend the Republic, to defend the people's freedoms as well as their achievements towards democracy! Through the statements by the government and the Popular Front (parties), the people understand the graveness of the moment. In Morroco, as well as in the Canary Islands, the workers are battling, united with the forces still loyal to the Republic, against the uprising militants and fascists. Under the battlecry 'Fascism shall not pass; the hangmen of October shall not pass!' workers and farmers from all Spanish provinces are joining in the struggle against the enemies of the Republic that have arisen in arms. Communists, Socialists, Anarchists, and Republican Democrats, soldiers and (other) forces remaining loyal to the Republic combined have inflicted the first defeats upon the fascist foe, who drag through the mud the very same honourable military tradition that they have boasted to possess so many times. The whole country cringes in indignation at these heartless barbarians that would hurl our democratic Spain back down into an abyss of terror and death. However, THEY SHALL NOT PASS! For all of Spain presents itself for battle. In Madrid, the people are out in the streets in support of the Government and encouraging its decision and fighting spirit so that it shall reach its conclusion in the smashing of the militant and fascist insurrection.
Young men, prepare for combat! Women, heroic women of the people! Recall the heroism of the women of Asturias of 1934 and struggle alongside the men in order to defend the lives and freedom of your sons, overshadowed by the fascist menace! Soldiers, sons of the nation! Stay true to the Republican State and fight side by side with the workers, with the forces of the Popular Front, with your parents, your siblings and comrades! Fight for the Spain of February the 16th, fight for the Republic and help them to victory! Workers of all stripes! The government supplies us with arms that we may save Spain and its people from the horror and shame that a victory for the bloody hangmen of October would mean. Let no one hesitate! All stand ready for action. All workers, all antifascists must now look upon each other as brothers in arms. Peoples of Catalonia, Basque Country, and Galicia! All Spaniards! Defend our democratic Republic and consolidate the victory achieved by our people on the 16th of February.
The Communist Party calls you to arms. We especially call upon you, workers, farmers, intellectuals to assume your positions in the fight to finally smash the enemies of the Republic and of the popular liberties. Long live the Popular Front! Long live the union of all anti-fascists! Long live the Republic of the people! The Fascists shall not pass! THEY SHALL NOT PASS!
