https://www.wsws.org/en/articles/2008/12/bgnl-d27.html

Trotsky worked within the Comintern, which worked within the Trade Union Congress, which worked within the Labor Party, which worked within the Parliament, the Kingdom, and imperialism in general. Lenin broke off all such alliances in the run-up to the Russian Revolution. Sylvia Pankhurst had urged the same position. Trotsky, fearing Stalin’s calling him a Left Communist or Infantile, isolated Sylvia Pankhurst, allowing the Labor Party, the police state and Stalinism to break up their publications and organizations. The same fate would befall the Left Opposition and the United Opposition. Lenin’s decision to break with the Left Communists had a terrible impact on the future of the revolution. Trotsky’s decision to follow along sealed the fate of the world revolution and gave imperialism the opportunity to restore its war-machines and go to war once again.

Lenin’s decision must be traced to his fear of isolation within the leadership of the Soviet Union due to the deteriorating economy and the possibility of an right-wing bureaucratic uprising against the new state and the newly established Soviet system. In reacting with fear, he gave that right-wing bureaucratic uprising the power by other means, allowing it to dominate the Comintern and the world revolution against capitalism while it isolated the Left Communists, the Left Opposition, and later the United Opposition.