Moscow and Riyadh have reportedly been negotiating for several years now about the Saudi purchase of Russian air-defense missile systems, but so far this has not yet occurred. Several analysts, including myself, have argued that Riyadh has made clear to Moscow that it will not proceed with any large-scale arms purchase from Russia unless Moscow scales back its arms sales to Iran.

But Russia’s relationship with Iran may not the most important factor in Saudi-Russian arms negotiations, and the delay in reaching a deal on air-defense missile systems in particular may  be because of other reasons.

Moscow’s delay — purportedly for “technical reasons”— in delivering S-300 air-defense missile systems to Iran has been attributed to pressure from Saudi Arabia, the United States and Israel. The deterioration in Russian-Iranian relations over the past few months, however, may now be a more important cause for the delay.

Unlike the West, the Kremlin immediately congratulated Iranian President Mahmoud Ahmadinejad after he was re-elected in June in a vote that was tainted by allegations of fraud and falsifications. Ahmadinejad, however, found a strange way to return the favor to Moscow: He rejected the International Atomic Energy Agency proposal for Russia and France to enrich Iranian uranium to medical grade. Thus, he increased the international community’s concern that Tehran is trying to develop a nuclear weapons program.

Furthermore, in December and January, Ahmadinejad called for Moscow to provide compensation to Tehran for the Soviet occupation of northern Iran during World War II. Moscow, of course, as a matter of principle does not apologize or provide compensation for any action it took that contributed to the defeat of Adolf Hitler.

In addition, on March 8 Tehran gave all Russian commercial pilots working in Iran two month’s notice to leave the country. Tehran, for its part, is angry that Russian officials are openly saying how increased United Nations Security Council sanctions against Iran may be needed if it does not increase cooperation with the international community on the nuclear issue.

Whatever the cause of Moscow’s previous delay in delivering S-300s to Iran, Tehran’s recent actions have only increased the likelihood of further, perhaps indefinite, delays. Russian’s frustration with Iran has grown so strong that Moscow hardly needs to be persuaded by Riyadh or anyone else to withhold delivery of the S-300s to such a difficult customer.

But if differences over Iran are not delaying the Saudi purchase of air-defense missile systems from Russia, then what is? One theory is that Riyadh does not want to buy the S-300 but the newer and more powerful S-400 instead. Indeed, the Saudis always want to buy the newest weapons systems and can obviously afford to do so. But while the Russian defense industry would be happy to sell the S-400 as well as other modern weapons systems to Saudi Arabia, Russia’s air-defense force wants to acquire all S-400s produced in the next few years for its own long-neglected needs.

Some Russian sources also see pressure from Washington as a reason why Riyadh has not yet signed a major arms agreement with Moscow. But this is unlikely. If anything, the ongoing tensions in U.S.-Saudi relations — as well as the Saudi desire to be seen as independent from Washington —would push Riyadh more in the direction of buying weapons from Russia. In addition, U.S.-Saudi tension is only likely to increase if the drawdown of U.S. forces in Iraq results in Saudi support for Iraqi Sunnis in response to the anticipated increase in Iranian support for various Iraqi Shiite factions.

Thus, it seems that a major Saudi weapons purchase from Russia is highly likely. It is just the timing that is in doubt. If this is the case, then this suggests that Saudi Arabia is far less concerned about whether Iran acquires S-300s to defend itself against Israel or the United States so long as Riyadh can acquire S-400s to protect itself from Tehran.

Mark N. Katz, professor of government and politics at George Mason University in Fairfax, Virginia, specializes in Russian foreign policy in the Persian Gulf.