So, I would actually disagree with the assumption that the Second Spanish Republic was a Stalinist puppet state in any way at any time. From a political perspective, the ruling figures of the republic were not Stalinists. While communists were allowed to politically participate, they were by no means the dominant political force. It was rather the liberal republicans and socialists who held most of the power. The republican government very much fell in line with what were called "popular front" governments that had been in and out of power in the first few decades of the twentieth century in countries like France most notably but also throughout the world. These popular front governments were mostly made up of centrists and leftists, specifically liberal republicans, libertarian socialists, and communists. It could even be argued that Franklin Roosevelt's New Deal Coalition constituted a Popular Front government. It would be foolish to say that Stalin had absolutely no indirect influence over republican Spanish politics at the time given the inclusion of communists on a political level, but it would be equally foolish to say that the republican government in Madrid directly received their marching orders from Moscow like the postwar Soviet satellite states.

The direct methods of influence received from the Soviet Union by the republican government was during the civil war in the form of military supplies. Indeed, the Soviet Union was the largest governmental supplier of arms to the republican forces during the Spanish Civil War. Hundreds of tanks and other combat vehicles, hundreds of thousands of small arms, as well as millions of shells and bullets were covertly sent to Spain by the Soviets with a price tag of about $500 million. It is also estimated that 700 Soviets at any given time were present in the country throughout the conflict. But to claim that this military support from the Soviets made the republicans obedient to Stalin is somewhat dubious. Yes, they were indebted to Stalin and the Soviets, but they were indebted to them in the same way that they were to so many others. In terms of official governmental support, Mexico was one of the most vocal in their support for the republican government. Furthermore, the government of Poland was only second the Soviets in terms of the amount of arms sold to the republicans. France, then ruled by a Popular Front government under Prime Minister Léon Blum, sent the Spanish republicans aircraft and pilots despite their being party to a non-intervention agreement with the British. Furthermore, the Lincoln Battalion, comprised of American volunteers who shipped themselves off to Spain to aid the republicans, was by itself made up of about 2,500 men and women, far outnumbering the amount of tank engineers and other soldiers that Stalin sent. This was supplemented by an estimated $2 million raised in the United States alone to aid the republican cause.

Therefore, while the Soviets were the largest supplier of arms to Spain, the republican Spaniards cannot really be considered pawns of Stalin in the global political game. Stalin's goal in supporting the republican Spaniards was not to form a Soviet-style puppet state like we would see after the Second World War, but rather it was an opportunity to keep communists politically enfranchised as well as to keep fascists like Franco at bay. Keeping a liberal government that tolerated the left would have maintained the European political power balance in favor of the Soviets. I cannot think of many scholars who would dare refer to the republican government in Spain as totally communist, much less Stalinist. If you want to read more about this, you can check out The Spanish Civil War by Hugh Thomas. On the broader topic of international intervention in the Spanish Civil War, you can look at Arms for Spain by Gerald Howson, while Ángel Viñas's article "Gold, the Soviet Union, and the Spanish Civil War" published in European History Quarterly is good for specifically the Soviet contributions. But that only barely scratches the surface, as there is so much literature about the Spanish Civil War.