IOTL, the Soviets were rather uneasy at the rise of Khomeini in Iran. His ideology, some in the Kremlin felt, could possibly ignite the Muslim nations in the southern Soviet Union. So, could the Soviets attempt an invasion of Iran so as to install an openly Soviet-friendly, socialist government? If necessary for this scenario to come about, let us say that the Soviets don't invade Afghanistan.
Is this situation plausible, and what would its effects be?