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?
The Americans send in their rapid deployment force that President Carter had authorized at the end of 1970's and activated at the time of the hostage crisis. Its purpose was to intervene 'out of NATO area' precisely in order to fight a Soviet invasion of the region.
You either have a Mexican stand off around Abadan with a UN sponsored ceasefire and a division of Iran into spheres of influence or a nuclear war.
Afghanistan didn't cause a wider war because it was/is a country no one really wants or cares about.