The United States :)
Hope im right!
The answer is the truman doctrine
In the late 1890s, the Foreign Office in London came to regard Germany as the main threat to the European balance of power and British imperial hegemony around the globe. This perceived German threat required a substantial modification of British diplomacy in other parts of the world and was instrumental in the British Foreign Office’s decision to reconsider its policy of rivalry with Russia, despite the Government of India’s continued concern with the Russian threat to the security of British India. Attaining Russia’s friendship became a primary objective of the Conservative British foreign secretary, Lord Lansdowne (1900-5), who initiated the talks for an Anglo-Russian understanding. However, it would be Lansdowne’s Liberal successor, Sir Edward Grey (1905-16), who finally managed to reach a formal accord with Russia in August 1907. By the time of the outbreak of the Persian Constitutional Revolution in 1906, London’s rivalry with Berlin had resulted in the abandonment of the British policy of “Splendid Isolation,” which precluded Britain’s participation in European alliance systems. Britain was now actively pursuing formal friendship with Russia in the European arena of balance of power and attempting to resolve the century-old Anglo-Russian rivalry in Central Asia and Persia. After the outbreak of the Constitutional Revolution in Persia, the British desire for cooperation with Russia placed the Foreign Office in London on a collision course with the Persian nationalist and constitutionalist reformers, many of whom initially looked to Britain for diplomatic assistance in countering overt Russian support for the Persian autocracy. After the conclusion of the 1907 Anglo-Russian Agreement, the British Foreign Office adopted a policy of ample tolerance towards Russian aggression in northern Persia and St. Petersburg’s efforts to obliterate the Persian nationalist/constitutionalist movement, despite periodic objections from the Government of India to London’s policy of appeasing Russian ambitions in Persia.
From 1907 until the outbreak of the First World War, British policy in Persia consisted of extensive cooperation with Russia, to the point of legitimizing Russia’s repeated violations of Persian sovereignty and substantial military presence in northern Persia. In the process, the British Foreign Office abetted Russia in undermining the Persian Constitutional Revolution in December 1911. After the outbreak of the First World War, Britain and Russia abandoned all pretense of respect for Persia’s sovereignty, jointly occupying that country under the pretext of countering German and Ottoman anti-Allied operations in Persia, despite Tehran’s declaration of neutrality in the war. The Bolshevik seizure of power in Russia in 1917 resulted in the cessation of Anglo-Russian friendship in general, and Anglo-Russian military and diplomatic cooperation in Persia in particular. With the withdrawal of Russian forces from Persia, already initiated after the March Revolution in Russia, the subsequent Bolshevik renunciation of the 1907 Agreement, and outbreak of military hostilities between Britain and the Bolshevik government after 1918, Britain attempted to establish its absolute imperial hegemony in Persia: first, through the abortive Anglo-Persian Agreement of 1919, and later by sponsoring the 1921 coup d’etat led by Rezā Khan and Sayyed Żiāʾ-al-Din Ṭabāṭabāʾi.
