I do not know what Trotsky in power would have done about Spain in the 1930's, but his judgments on it in OTL were certainly questionable. To quote an old post of mine:
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... Here I want to ask: What if the POUM really *had* been Trotskyist and had followed his advice? In other words, how well does Trotsky's evaluation of the situation in Spain hold up? My answer is, Not very well.
Trotsky's basic problem is that he saw things in Spain through the lens of Russia in 1917. For example, for years he insisted on the slogan of soviets in Spain. As Robert J. Alexander notes, "The soviets had arisen in Russia largely because of the absence of well-established trade unions and mass-based workers parties; whereas in Spain there existed strong trade unions and political organizations with which several generations of workers had been affiliated, and which both exerted discipline over their own followers, and were not at all ready to get out of the way to make room for some new type of workers organization." This was above all true of the anarchist workers of the CNT which was very strong in Catalonia. Trotsky seems to have viewed the CNT as no better than the Mensheviks, and was constantly urging the POUM on to policies that would have alienated the CNT workers.
For example, take the POUM's *limited* participation in the Popular Front at the time of the 1936 elections. It supported the Front purely as an electoral alliance at a time when failing to do so would have alienated the entire Spanish Left. Even the Anarchists gave up their traditional electoral abstention because a Popular Front victory was the only way to get an amnesty for political prisoners. But to Trotsky, even this limited support for the Popular Front was "class collaboration"--again based on Russia in 1917. He seems to have paid no attention to the fact that after the election Maurin announced POUM's withdrawal from the Popular Front.
Above all, there was this difference between Spain and Russia: In Russia, the Bolsheviks could win popularity by denouncing the increasingly unpopular war with Germany. In Spain, the war was with Franco, and it was essential not to alienate the non-POUM workers and peasants who made winning this war their first priority. Trotsky's reply to this was that the only way to win the war was through social revolution--agrarian reform, etc. But this ignored that, as Alexander notes, in Spain a social revolution *had already occurred* and the task was to defend it: "in Spain the peasants under anarchist and Socialist leadership had seized the land in the first days of the Civil War. Similarly, he seemed to give no recognition to the fact that the workers in Catalonia and many other parts of Republican Spain had seized control of their factories, railroads, and utilities the day the military revolt had been suppressed.
"To a large degree, therefore, the workers organizations in Spain — whether anarchists of the CNT-FAI, POUMists or left-wing Caballero Socialists — were faced with the problem of how to defend the revolutionary conquests they had made during the first days and weeks of the Civil War. Their choices were difficult in the extreme. They may have erred from time to time, although it is by no means clear that alternate choices would have brought any better results for them. But in any case, Trotsky tended to regard errors of judgment — if that is what they were — as "betrayal"...
An example of this is Trotsky's denunciation of POUM's September 1936 decision to join the Catalan government. This may indeed have been a mistake. "But in utterly condemning that decision, Trotsky certainly gave no weight to the fact that to have stayed apart from the Catalan government would have been for POUM to isolate itself totally from the CNT-FAI under circumstances in which the CNT-FAI was the principal bulwark of the revolution in Catalonia. Trotsky seemed not at all to recognize the need for POUM to maintain a relationship with the CNT-FAI which might permit it to influence the thinking and actions of the anarchists, not only to defend the revolution, but to defend the very existence of POUM itself in the face of the onslaught of the Stalinists..."
In short, "The POUM, led largely by Trotsky's ex-comrades, was not at all in the position of the Bolsheviks of 1917. Rather than being able to lead a crusade against an unpopular war and for yearned-for reforms, it had to find ways to defend a revolution which had already occurred — in the face of overwhelming pressures from other supposed "Marxist-Leninists" against it — and to do so without endangering the prosecution of a war the winning of which everyone (including Trotsky) agreed was the sine qua non for revolution of any kind. They may have made errors in judgment, but they were certainly not "betraying" the revolution..."
Of course one could go back further and note that apart from mistaken analogies with Russia, Trotsky also generalized from the experience of other countries, ignoring Spanish conditions. For example, 1934-36 was the period of the "French turn" in Trotskyist politics.
https://en.wikipedia.org/wiki/French_Turn Trotsky ordered his French followers to join the French socialist party, the SFIO. He mistakenly thought that this tactic could be applied to Spain as well and sought to have the Spanish Trotskyists of ICE join the Spanish Socialist Party (instead of merging with the BOC as in OTL). "In the Spanish case he not only completely swept aside the fact that the ICE leaders and members were almost unanimous in opposition to the tactic, but also the fact that the Spanish Socialist Party was very different from that of France. Whereas the French party allowed formal factional groupings within its ranks, that of Spain did not. The Trotskyists would not have been admitted to the PSOE if they had tried to go in, as the International Secretariat described it, "with their flags flying."
"To greatly understate the case, Leon Trotsky did not show himself at his best in dealing with his Spanish followers. Certainly the totality of the blame for the ultimate break between them did not rest on his shoulders, but most of it did. His dogmatism, his lack of knowledge about the situation, his ultimate insistence on obedience on the part of his supporters all created a gulf which proved unbridgeable. But perhaps had Leon Trotsky acted differently in this case, he would not have been Leon Trotsky."
https://www.alternatehistory.com/forum/threads/what-if-the-poum-really-was-trotskyist.432701/