This article presents a unique source-critical and historical analysis, introducing previously unknown letters of Joseph Stalin discovered in the author’s personal archive into scholarly circulation. Utilizing these documents, the author reconstructs the fierce intra-party struggles within the Bolshevik leadership in 1925–1927 — a pivotal period that determined the outcome of the power struggle following Lenin’s death. The newly published letters, addressed to V.M. Molotov, A.I. Rykov, and other leaders, shed new light on the mechanisms of political decision-making, Stalin’s tactics in defeating the united opposition (Trotsky, Zinoviev, Kamenev), and his assessments of international crises (the Istmen affair, the British general strike, the Chinese question). Particular attention is devoted to dramatic episodes: the Lashevich affair, the “war scare” of 1927, and the creation of the opposition’s underground center. The published letters not only complement the well-known Stalin–Molotov correspondence but also demonstrate the evolution of Stalin’s policy from a carrot and stick approach to the complete suppression of all factional dissent. The publication of the letters is accompanied by extensive historical context, allowing the reader to gain a deeper understanding of the reasons behind the eventual transition from the new economic policy (NEP) to forced industrialization and the tightening of the political regime.