The Moscow trials occurred in the last phase of the conflict between the Party leadership and the "oppositions" which had arisen in the twenties. It is therefore relevant to recall very briefly a few points about these oppositions.
During the second phase of Lenin's illness, in 1923, Kamenev, Zinoviev and Stalin had paramount influence in the Politburo and were opposed within that body by Trotsky. In the fall of 1923 this conflict became public. Trotsky and his group demanded a greater degree of democracy within the Party, a larger participation of youth in its leadership, and a change in economic policy. Ivan Smirnov, Mrachkovsky, Ter-Vaganian, Dreitzer (who became defendants in the trial of 1936), Piatakov, Radek, Serebriakov, Muralov, Drobnis (who were defendants in the trial of 1937) were active in this opposition. It was ended by the Party Conference of January 1924 and the 13th Party Congress of May 1924: overwhelming majorities of delegates--setting the pattern for many occasions of this kind during the following years-- condemned the deviators. A number of prominent oppositionists were transferred to less influential positions or to geographically remote (e.g., diplomatic) posts.
In the fall of 1924 Trotsky returned to the attack. He charged the ruling group--in particular, Kamenev and Zinoviev--with responsibility for the defeats suffered by the Comintern in Germany and Bulgaria in the preceding year, and recalled past conflicts between Lenin and the members of the triumvirate. Both his charges and the counter-attack against his conception of the "permanent revolution" were more violent than they had been a year before; and the facilities granted to the opposition in the ensuing "discussion" were more limited. Kamenev and Zinoviev apparently proposed to Stalin that Trotsky be expelled from the Party and imprisoned; but Stalin--using the technique of ostensible moderation which he maintained during the following years--refused. At a joint session of the Central Committee and the Central Control Commission in January 1925, Trotsky's opposition was brought to an end by the Party leadership. The limited and unavowed sanctions taken against major oppositionists were of the same order as those which had been applied in the previous year.