The Treaty on Friendship, Mutual Assistance and Postwar Cooperation between the Czechoslovak Republic and the Union of Soviet Socialist Republics signed on 12 December 1943 in Moscow had a fundamental impact on the orientation of Czechoslovak foreign policy at the end of the war and in the years that followed. At the same time, the lengthy negotiations in 1943, which ultimately resulted in signing the treaty in question, were one of the few moments during the war when Czechoslovakia became the object of an opinion clash between the Great Powers. In this study, which is based primarily on British and US documents (some of which have not been used before), the author analyses in detail the role of the Czechoslovak-Soviet treaty project in the policies of the two Western powers until the signing of the document, before assessing the impact of the treaty in concern on Czechoslovakia’s relations with the United Kingdom and the United States at the end of the war. He points out that neither the British nor the Americans were prepared to conclude a similar treaty with Czechoslovakia since both Western powers wanted the international security system to be based on foundations different from those which had repeatedly failed during the previous three decades. However, the signing of the Czechoslovak-Soviet Treaty dramatically reduced any chance for a federative or confederative arrangement in the region of Central Europe, as well as hopes for a multilateral treaty of alliance ensuring security in this region. For this reason, it was accepted without enthusiasm both in London and in Washington.