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In page History of Kazakhstan:

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This objective became particularly important after 1990, when it was learned that Mikhail Gorbachev had negotiated an agreement with the American Chevron Corporation to develop Kazakhstan's Tengiz oil field; Gorbachev did not consult Nazarbayev until the talks were nearly completed. At Nazarbayev's insistence, Moscow surrendered control of the republic's mineral resources in June 1991 and Gorbachev's authority crumbled rapidly throughout the year. Nazarbayev continued to support him, urging other republic leaders to sign a treaty creating the Union of Sovereign States which Gorbachev had drafted in a last attempt to hold the Soviet Union together.