I've been batting this around in my head for awhile, and it's still pretty rough. But I need something to get my mind off of waiting for the kid to come, so I'll pitch it.
This is from
Iranian F-14 Units in Combat, by Tom Cooper and Farzad Bishop:
Iran's inability to prevent overflights by Soviet MiG-25Rs is said by many Western observers to have been one of the main reasons why the country acquired the Grumman F-14. The truth is slightly different, however. Since the late 1950s, the Imperial Iranian Air Force (IIAF), in cooperation with the USAF, had been flying highly secret reconnaissance flights over the USSR. Initially fighter aircraft (even transports) were used, and several were shot down by Soviet fighters. After the first F-4s arrived, the IIAF also received some RF-4Es, and operations were intesified.
The Soviets were, understandably, concerned about Iran's massive rearmament and started their own reconnaissance missions over the country. IIAF interceptors, especially F-4Ds, repeatedly tried to catch the MiG-25Rs, but this proved a very difficult task as the routes flown by the overflying 'Foxbats' were carefully chosen.
The Shah was not interested in a direct confrontation with the USSR, so as the mutual airspace violations intensified, he offered to stop IIAF (and American) overflights if the Soviets would do the same. This offer was repeated, and turned down, several more times. The IIAF was therefore ordered to fly two or more missions over the USSR for every 'Foxbat' flight, initiating a 'tit for tat' campaign. But a more powerful weapon system than the Sparrow-armed F-4s was needed to prevent Soviet incursions.
That "more powerful weapon system" would be the Phoenix-armed F-14A Tomcat. And by the time of the Iranian Revolution in 1979, the Islamic Republic of Iran Air Force (IRIAF) had 79 purchased, of which 77 where operational.
At this same time, during the late 70s, the Soviet Union was extremely nervous of a South Asian alliance system, allied to the United States, forming on it's border. Consisting of Iran, Afghanistan, and Pakistan, the USSR felt that such a line of countries, with ICBMs and American aircraft based there, could cause serious problems should they come to blows with the West.
One such countermeasure was the propping up of a Communist regime in Kabul, and the subsequent involvement of ground troops in Afghanistan when that went south.
But, let's suppose for a moment that the Soviets went a different route. The shoring up of an unpopular regime in Afghanistan was not the only option. In 1979, before Afghanistan was demanding a fair amount of manpower (or would for three or four years), the Islamic Revolution would draw Iraq into invading it's neighbour. West German and Soviet pilots, in MiG-23s and -25s, flew from bases in Iraq to train against the newer Western equipment.
The Soviets, however, could have gone with another option. A bit more hawkish than the covert support they supplied, yes, but the chances were there (and the Pentagon made this very clear up until the late-80s) for a Soviet invasion of Iran.
The overflights and the Revolution provide us with sparks and logical reasons. Now...what would be the logical progression of such a war, and would the Soviets, once they decided to do this, want Iraq to invade as well?