It began as an army revolt supported by the Falange (fascist) and the Carlist(Catholic monarchist) militias.
Britain, France and the U.S. prevented arms sales to the republic and the French closed their frontier to them.
Many in those countries were sympathetic to the nationalists.
They Republic was wholly dependant on the Soviet Union for planes and tanks. This dependance allowed the communists, at the beginning a tiny and uninfluential group to grow in influence, by the end they almost completely controlled things (until a socialist/Anarchosyndicalist coup that ended the war).
Italy and Germany sent their proper airforces and in Italy's case entire divisions and the use of Submarines (even against British ships). The Germans also ferried the Army of Africa to Spain in the initial stages which gave the Republic less time to finish off isolated cities and garrisions still in revolt.
These backers also sent supplies on credit, whereas although they had the governments gold reserves the Republic was made to pay in advance. Also much of their supplies were sold by the Germans. Profiteers in the government split up the weapons and sold the low quality supplies to the Republic.
Also the Soviets tied the hands of the Republic which they insisted not offer Morroco independance (it would offend France
) which would have created a revolt in the prime recruiting area and original base of the Nationalists as well as depriving them of the Morrocan tribesmen who served in nationalist ranks.
Another problem was the influence of the "advisors" who insisted on outdated and just bad tactics, especially in the use of tanks becuase the proponents of better tactics had been convicted in a show trial, making his ideas "trotskyist plots".
The communist and NKVD ferreting out of "fascists" and "trotskyists" was bad for moral and killed many people. They also insisted all spectacular failures
Brunette, Teruel, and Ebro exhausted the republic and wasted its equipment) were the result of Fascists and Trotskyists treason and refused to learn from the mistakes although they were always the same:
1. The offensives would wait to capture minor positions that they could easily have bypassed until later giving time for the Nationalists to redeploy and halt the offensive, at which poing Germano-Italian air power gave them a pounding.
2. Then the Soviet habit of outlandish propaganda (they would claim victory prematurely and make to much of minor strategic objectives) would trap them into a keeping up a failed offensive. at which because of enemy air power they would be defeated at with huge material losses.
Anothe phenomena was that Soviet and Soviet trained pilots were generally extremely conservative, and tried to avoid daring action at the expense of operational effectiveness.
Anyway the British appeasement mongers kept the "non intervention" going among countries other thant the three primary interveners.
When Italian subs attacked British ships they didn't want to risk their futile attempts to draw Italy away from Germany, so they did nothing against the Italians.
At the end when the French became antsy the British actually told them that if French intervention turned into war with Germany
they would NOT support them.
Actually after the pathetic concessions in Czechoslovakia Stalin actually abandoned the idea of opposing fascism entirely, and support declined, although it was to late for the Republic at any rate.
International volunteers played a role in Spain too, many later serving in WWII.
At the end there were massacres and oppression for years against former residents of the Republic and those who fled to France en masse were placed in concentration camps where many of them died.