The following is based on the missions of Counterstrike and might contradict canon.
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Soviet activity in Siberia is increasing. A large shipment of atomic material is on its way through this area, and we need it stopped. Destroy all of the Soviet convoy trucks and prevent the Soviets from furthering their atomic testing.
Be careful of the convoy truck cargo -- atomic material is highly unstable.
The mission is complete when all the convoy trucks from each of the seven convoys have been destroyed.