Operation Prime Chance

An aerial view of the leased barge Hercules with three Mark III patrol boats and the tugboat Mister John H tied up alongside in the northern Persian Gulf.

In conjunction with Operation Earnest Will (Aug 1987 to June 1989) the United States tasked its Special Forces Command with deterring Iran from using naval mines to impede Gulf shipping. In a response to losses in its naval forces, Iran began a new strategy in the Tanker War by using various forms of naval mines to slow Gulf shipping. The use of mines goes back to World War I, with mines being a cheap and effective defensive weapon. Moored mines were common in World War II, yet mines do not need to be moored to be effective. A secondary use is strategic area denial. Placement of mines in strategic passages effectively and cheaply denies passage to ships. The effectiveness of an area denial strategy is increased when the party planting the mines has knowledge of the currents and eddies that are present in shipping lanes. By planting mines in currents that will take them into the shipping lanes the planter can lessen their chances of detection and retaliation (Larson et al. 2004, Krepinevich et al. 2003, Truver 2008). Seeding currents could even be done from one’s own territorial waters making efforts to deter the mining difficult. In order to disrupt Gulf shipping Iran began to use free-floating mines in the Gulf in mid-1987.

Operation Prime Chance was carried out by American Special Forces, with the goal of interdicting Iranian vessels that were seeding the Gulf with floating mines. While the re-flagging operations of Earnest Will were publicized, Prime Chance was secret (Cordesman & Wagner 1990, Crist 2001, Kelley 2007, Selby 1997). The plan was for Army and lesser numbers of Navy helicopters to interdict the mine laying vessels, which were in many instances revamped fishing vessels or the ubiquitous Arab Dhow. Using night vision devices the helicopter pilots operated from land, ships (Navy helicopters only) and barges (the Wimblown and Hercules) known as Mobile Sea Bases (MSBs). Attached to these mobile platforms were two or three US Navy Seal patrol boats that followed up on various contacts, boarded suspicious vessels and provided security for the bases. As floating bases the MSBs were moved from one part of the Gulf to another in response to mining activity. The operation lost its secrecy on September 21, 1987 when American forces staged an attack on the Iran Ajr, a small ship used by Iranian forces for mine laying. Attacking by air and then by sea the ship was seized, intelligence collected, and then the ship was scuttled. The incident demonstrated that Iran was indeed responsible for mining of the Gulf (Cordesman & Crist 2001, Kelley 2007, Selby 1997, Wagner 1990). Video footage of the Iran Ajr laying mines in the Gulf was broadcast worldwide and undermined the Iranian government instance that the mines were planted by Iraq and the United States in order to embarrass Iran.

Operation Prime Chance provided political cover for the re-flagging operation given the mines that were hit in the course of getting the operation going and the lack of mine clearing vessels in the American Navy’s inventory. Having to rely upon third parties to clear the mines was an embarrassment to the Reagan Administration and Operation Prime Chance was a way for the Untied States to stop the mining of the Gulf in a secret and effective manner. Politically the success of the Iran Ajr seizure enabled the Administration to demonstrate that Iran was behind the mining of the Gulf. Undercutting the credibility of the Iranian government at this juncture was important for the United States and the Gulf States because both Iraq and Iran were almost to the point where they were ready to call for a cease-fire in their eight year old war.

For Iran the mining of the Gulf was a rational strategy used to intimidate the Gulf States from supporting Iraq. The American response was perhaps stronger than anticipated and the operations were discovered and publicized before the Iranian strategy had a chance to work. In short, the United States had thwarted the strategy. However, it should be noted that the Iranian government had been seeking a way out of the increasingly costly war. While the war was initially popular, as time dragged on and causalities mounted fewer families were willing to allow their sons to join the Basij, whose human wave tactics left scores dead or maimed. There was political will to end the hostilities, yet the tipping point had not been reached at this point in time.

The various military operations undertaken by the United States demonstrated not only how important the United States regarded free navigation of the seaways but it also revealed how it sought to attempt to use what little leverage it had to get the Iranians and Iraqis to accept some type of cease-fire in order to lessen tensions in the Gulf. The last great incident involving the United States and Iran in the evolution of this low-intensity conflict was the shooting down of Iran Air Flight 655 on Sunday July 3, 1988, by the cruiser USS Vincennes killing all 290 passengers and crew aboard the Airbus A300B2 (Flight 655) (Cordesman & Wagner 1990, Crist 2001, Kelley 2007, Selby 1997).

The Vincennes was providing cover for a heavy lift vessel that was carrying the Roberts back to the United States for repairs. Flight 655 took off from Bander Abass bound for Dubai. The flight path skirts the northern part of the Straits of Hormuz, crossing the shipping lanes that lie closer to Iran than the Emirates, and then into Emirates airspace before landing at Dubai. The airport at Bander Abass is a joint military-civilian airfield, which may have been a factor in the American ship determining that the intent of the aircraft was hostile. Early in the morning the Vincennes’ helicopter received small arms fire from an Iranian gunboat. As the Vincennes moved to engage the gunboats, they noticed the track of Flight 655 was on a bearing directly toward their position. Flying in a regular commercial air corridor Flight 655 took off then climbed to altitude. The Vincennes officers felt that the aircraft could be an F-14 armed with bombs using commercial routes to mask their approach to attack the American cruiser. The warship fired two missiles that destroyed the airliner killing all 290 passengers and crew (Cordesman & Wagner 1990, Crist 2001, Kelley 2007, Selby 1997).

Various factors could have contributed to the tragedy. One factor was the apparent confusion or ignorance of the commercial air routes over the Gulf by the Vincennes crew. Moreover, American warships had no communications that could monitor normal commercial radio frequencies. The only frequencies they monitored were emergency frequencies (Cordesman & Wagner 1990, Crist 2001, Kelley 2007, Selby 1997). The airliner may have thought that the calls to change course were aimed at an Iranian P-3 Orion anti-submarine warfare aircraft operating in the area. The P-3 is capable of firing anti-ship missiles. With the P-3 in the area, the warship was concerned about having not one but two aerial threats while seeking to disengage from the Iranian speedboats. Perhaps the greatest factor at the heart of the tragedy was the sophistication of the Aegis combat system, a computerized system designed to engage aerial, surface, and sub-surface threats automatically and simultaneously. Initially conceived to protect carrier battle groups in open waters, Aegis was out of its element in the confines of the Persian Gulf where hostile, friendly, and neutral aircraft and ships interact. Another contributing factor was lack of training for the crew, who had little experience working with the Aegis system.

The reported aggressiveness of the Captain of the Vincennes was a concern. Captain William Rogers was reportedly more aggressive than most captains to the point of actually chasing Iranian speedboats with a billion dollar cruiser not designed for that mission (Cordesman & Wagner 1990, Crist 2001, Kelley 2007, Selby 1997). We do note, however, that Rogers waited until what he thought was a hostile aircraft reached within 15 miles of the Vincennes to fire when the rules of engagement called for firing on a hostile aircraft at a range of 20 miles. Given the close quarters of the Gulf, lack of training, the lateness of the flight (27 minutes), other aircraft in the area, unfamiliar computerized combat systems (Dotterway, 1992) and the failure to properly classify the Iranian plane as ascending and the American plane as descending combine to became a deadly mix.

The importance of Flight 655 and Operations Praying Mantis, Earnest Will, and Prime Chance (among others) was that Ayatollah Khomeini reasoned that the game of brinksmanship his government was playing with the United States would eventually lead to an all out American attack. Such an attack would devastate Iran, especially if key defensive installations such as the Silkworm Missile launchers in the Straits of Hormuz or major air and naval bases as are in Bander Abbas were destroyed. Most frightening was the very real possibility that the United States would attack the Kharg or Larrak Islands where Iranian petroleum exporting facilities were located. Loss of revenue would severely undermine the government’s finances and perhaps even undermine the revolutionary institutions that had been put in place. On the other hand, the coalition that put Khomeini in power had grown weary of the strict edicts on dress, speech, and travel. The war, while disproportionally shouldered by the lower classes that tended to support the government more vigorously, would have been in danger of losing subsidies and transfer payments that oil revenue provided. Thus, the social bases of the revolution could have been threatened. The more affluent Iranians, who had not seen financial or social gain under the new regime, would have even less reason to support the regime and indeed may have openly opposed the government if the conflict widened or oil revenues were curtailed. To be sure, Khomeini was confronted with losing his main base of support or even risking open rebellion if the United States engaged in open warfare against Iran and Iranian interests.

Given this situation, he decided—in what must have been a galling decision—to seek a cease-fire with Iraq. Perhaps the best way to look at this reversal is the fact that it was the sum total of small defeats that signaled that the United States was serious in its threats and that it would attack Iranian forces at will. The destruction of Flight 655 may have signaled an escalation by the United States that Iran could not counter, save for closing the Straits of Hormuz, which would certainly result in American military action. If the Iranians saw Flight 655 as a deliberate signal that the United States would now engage civilian targets then Khomeini would be correct in seeking an end to hostilities with both Iraq and the United States. This is in fact what he did, thus saving his governing coalition and setting the stage for a redirection of oil revenues to other economic sectors. If the shooting down of the airliner was a mistake as claimed by the American Navy then Khomeini made a practical decision in seeking a cease-fire with Iraq and de-escalation with the United States, given the increasingly severe American military actions as well as the dwindling of Iranian military assets available to confront the United States.

In sum, Khomeini’s decision to disengage was made on practical political, economic, and military basis; thus, ensuring that his revolution and institutions would survive intact. Indeed recent evidence suggests that Khomeini concluded from the various American military actions, in particular the destruction of Flight 655 that the United States had decided to undertake unlimited military actions against Iran, given the slightest provocation. Knowing that American forces took great care not to involve civilians, the downing of the airliner must have lead the Iranians to believe that the United States would now target Iranian civilians. Having undergone the “War of the Cities” missile attacks in 1985, the leaders in Tehran were clearly concerned that the United States could mount a much more devastating attacks than had Iraq, and they were concerned that their defenses were not capable of defending Iran. Thus, given months of constant and increasing tension with the United States, which culminated in the destruction of Flight 655, Ayatollah Khomeini concluded that full scale war with the United States was a very real possibility. Thus, he decided that it was time to end the conflict with Iraq and deescalate tensions with the United States (Wilson Center).


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