Although there were many factors that contributed to the failure of the Left Opposition to achieve power and the rise of Stalin, overall I agree with this statement. Despite not being the sole cause, I believe it to be the main cause. In this answer I will discuss the failure of Trotsky, Zinoviev and Kamenev to take notice of Stalin and his actions, along with other reasons such as the lack of opposition facing Stalin, Stalin’s personal strengths and the Left’s flaws.
Trotsky failed to notice what Stalin was capable of and simply dismissed him as a “grey blur” of not much importance to the party. The suggestion of Stalin being a threat would have been something of a bad joke to Trotsky who saw Stalin as intellectually inferior.
However, you could argue, what worsened this indifference to Stalin was Trotsky’s lack of motivation in his political career. He could have rallied support from soldiers he led in the red army or younger party members who looked up to him, but he saw this kind of mundane political canvassing as below himself. His aloof and big headed personality also put people off him and so didn’t encourage people to commit their loyalties in him the same way they did with Lenin. From this perspective, you could say it wasn’t the Left’s underestimation but Trotsky’s own personal downfalls that lead to the Left’s failure to achieve power.
Zinoviev and Kamenev, like Trotsky, underestimated Stalin, viewing him as less of an intellectual, and more of someone to be controlled to their advantage. They both ignored Stalin’s scheming to build his own power base while they were wrapped up in their campaign against Trotsky. Meanwhile, Stalin, being a member of all key bodies of the party was able to quietly gain loyalty from the people he placed and promoted. Zinoviev and Kamenev only noticed when it was too late and Stalin had the entire party behind him.
You could argue that this was not the Left’s underestimation but their fault for trying to tear each other apart. Or that Stalin was rising to power because others were noticing the fact he seemed to be doing well and backed the winning horse to ensure their own security.
Zinoviev, Kamenev and Trotsky all underestimated Stalin when they turned down the opportunity to…