Coming in the wake of Russian warships passing through the Panama Canal and visiting Cuba, conflicting reports that Moscow intends to sell an advanced anti-aircraft missile system to Iran are ratcheting up tensions with the United States. But more worrisome still is the heightened prospect of a preemptive Israeli air strike against Tehran's nuclear infrastructure before the S-300 system would become operational.

On Sunday, Iranian official Esmail Kosari seemingly confirmed earlier rumors of the purchase, telling Tehran's IRNA news agency, "After a few years of talks with Russia, now the S-300 system is being delivered." But the next day, the Russian agency responsible for monitoring international defense cooperation denied plans for imminent deliveries of the S-300 to Iran, claiming the Iranian's revelation "does not correspond to reality." Yigal Palmor, a spokesman for Israel's Foreign Ministry, also said a senior Russian official had "told Israel that the new report about delivery of the S-300 was false."

As the AP reported Tuesday, despite the Russian assurances American officials believe the sale of the SA-10 (as it is known in the West) is going forward. While protesting that the sophisticated anti-aircraft system would pose a threat to U.S. forces in Iraq and Afghanistan, Washington's bigger concern is the prospect of dramatically improved air defense for the Iranian nuclear program. As the Washington Post detailed:

Israel and the United States fear that, were Iran to possess S-300 missiles, it would use them to protect its nuclear facilities, including the uranium enrichment plant at Natanz or the country's first atomic power plant now under construction at Bushehr by Russian contractors. That would make any potential military strike on the Iranian facilities much more difficult.

Make that much more difficult.

Following Tehran's recent acquisition of Tor M-1 surface-to-air missiles, the S-300 system would alter the calculus for the Israeli and American defense planners contemplating a strike against supposed nuclear weapons-making targets in Iran. As the Jerusalem Post noted in August:

[The S-300] is one of the most advanced multi-target anti-aircraft-missile systems in the world today and has a reported ability to track up to 100 targets simultaneously while engaging up to 12 at the same time. It has a range of about 200 kilometers and can hit targets at altitudes of 27,000 meters.

To be sure, the Israelis aren't standing still. In August, an Israeli defense official claimed his country was already developing electronic warfare devices to "neutralize" the S-300. Israel has also purchased 90 long-range F-16I fighter planes which can carry enough fuel to reach Iranian targets. And in June, Israel carried out a massive aerial exercise in the Mediterranean with 100 F-16 and F-15 fighters, a maneuver American defense officials viewed as part warning and dress rehearsal.

As for the outgoing Bush administration, its time and options are limited, if not its preferences. , The New Yorker's Seymour Hersh reported that as early as April 2006, the Bush administration was "planning a massive bombing campaign against Iran." In late 2007, Hersh claimed, the President gave the green light to escalating American covert operations within Iran. (That charge was denied by U.S. Ambassador to Iraq, Ryan Crocker.) And despite the opposition of Defense Secretary Robert Gates and Joint Chiefs Chairman Michael Mullen, Vice President Cheney is said to prefer that the U.S. and not Israel strike Iran, as "we'll be blamed anyway." (Hersh also reported in July that Cheney hoped to trigger a confrontation with Tehran by staging a shooting incident in the Strait of Hormuz with PT boats manned by U.S. Navy Seals dressed as Iranians.)

All of which mean Barack Obama can't enter the White House a moment too soon. Obama has pledged a diplomatic offensive to engage Tehran over its nuclear program. (Last week, the Washington Times reported that Obama plans to name an envoy for outreach to Iran.)

But he won't have much time to alter the trajectory of events in the Persian Gulf. The impasse with Medvedev over the U.S. missile defense system is aggravating Russia's defense of its aid to the Iranian nuclear program. While a recent National Intelligence Estimate (NIE) concluded that Iran abandoned its plans for nuclear weapons in 2003, the IAEA announced in November that Tehran now has enough enriched uranium (if not the know-how) for one atomic bomb. And while upcoming elections in Israel may delay any decision to launch an assault against Iranian nuclear facilities before a deployment of the Russian S-300 systems is complete, a victory by the hard-line Likudnik Benjamin Netanyahu makes that prospect more likely.

(This piece is crossposted at Perrspectives.)