Trotsky viciously attacked the Soviets not only for their non-involvement in Spain, but also their continued commercial ties to Germany and Italy. [1] Writing “We now see the Capitalist roaders in Moscow collaborating with Fascists to crush the Proletarian revolution in Spain. The blood of workers and peasants are on their hands.”
In the fall of 1936, Trotsky announced the creation of the fourth internationale as an alternative to the Comintern, declaring “The working class of the world will not abandon our Spanish comrades in their historic struggle against Fascism”. He lent his prestige and pen to the effort to recruit “international brigades” to join the fight.
The material results of those efforts were quite modest: Trotsky’s arrogance and tendency to insult others made him unpopular even among leftists who agreed with his ideals. Some have even claimed that Trotsky’s involvement might have hurt the Republican effort through adding to infighting more than helped it.
Still, in a few months time several hundred leftists with varying degrees of Trotskyite sympathies [2] were equipped, given basic training, and sent to Spain to join the fight. Trotsky boasted of this, in the words of one commentator, “as though five-hundred thousand rather than five-hundred men have arrived in Spain”. Trotsky soon proclaimed that this was evidence that the fourth internationale and not the Comintern was the “real'' leader of the Socialist movement.
The effect of this was to provoke an outsized reaction from the Soviet leadership. Trotsky’s propaganda effort made the Politburo seriously worry about the ideological legitimacy of Soviet Socialism. As Premier Rykov said in a Politburo meeting: “We cannot allow Trotsky to seize from us the leadership of the revolution: we must demonstrate to the world that the Soviet state has not abandoned the struggle”. The decision was therefore made to intervene in Spain.
Suggestions that Trotsky be simply “eliminated” by the NKVD was unanimously rejected with “The party would abhor shedding the blood of an old Bolshevik”. [3]
At the Defense Commissariat, Marshal Mikhail Tukhachevsky saw a golden opportunity. He and his staff had been developing the doctrine called “Deep Battle” which foresaw the heavy use of armored force and manoeuvre formations to break enemy lines. Spain looked to be a battlefield in which his theories could be tested, possibly against Fascist German and Italian “volunteers”. He had a regiment of tankists and their equipment ready to be sent to Spain, if only this can be done covertly.
Spain was not the only place where the red army would soon be tested, as an old rival starts a new war on the other side of the world.
[1] Who are intervening in Spain as per otl
[2] Mostly left-wing German/Italian exiles
[3] Stalin was the -only- party leader willing to break that taboo otl