This nuke deal with Iran had better work. Because the Kremlin is lifting a ban on selling a powerful air defense system to Iran that would render an airstrike on Tehran’s nuclear weapons facilities nearly impossible.

The delivery of the new weapon, called the Almaz-Antei S-300PMU-1—known as the SA-20 Gargoyle in NATO parlance—would effectively force the U.S. to rely on its small fleet of stealth aircraft to strike targets inside Iran in case the mullahs make a dash for the bomb. But even those aircraft might have a difficult time.

“This would be a huge deal depending on where they [the S-300s] are based…The Persian Gulf would be an interesting place to fly,” said one senior defense official with experience on multiple stealth aircraft types. “These new [surface-to-air missiles] change the whole complexion…It’s a big move.”

According to a report from Russian state media, Russian President Vladimir Putin signed a decree on Monday that would allow the sale of the fearsome S-300 air defense system to Iran.

“[The presidential] decree lifts the ban on transit through Russian territory, including airlift, and the export from the Russian Federation to the Islamic Republic of Iran, and also the transfer to the Islamic Republic of Iran outside the territory of the Russian Federation, both by sea and by air, of air defense missile systems S-300,” reads the Kremlin statement, according to RIA Novosti.

The U.S. government has lobbied Russia hard for years to prevent the sale of the S-300 to Iran. In 2010, convincing Putin to suspend the sale of the S-300 to Iran was heralded as a major foreign policy coup by the Obama administration. In many ways, it was one of the central achievements of the so-called reset in relations with Moscow, said Heather Conley, a Russia expert at the Center for Strategic and International Studies in Washington.

Since then, of course, relations with Russia have cooled to nearly Cold War levels of hostility. Making life difficult for American policymakers is once again a Kremlin priority. “Mr. Putin’s policies are not designed to assist the West or to make our jobs and ability to affect policy much more difficult,” Conley said. “It’s also a reminder to Washington and other Western capitals that they have some cards to play here.”

Another factor that might be motivating Moscow is that with the Russian economy in shambles, Moscow needs all of the economic stimulus it can get. The missile deal with Iran would reportedly net Russia more than $800 million.

Last year, analysts predicted that if the U.S. sanctions of the Russian economy grew too tight, the Kremlin would respond by selling S-300s to Tehran. “I could see as part of this deal [between Tehran and Moscow] that they would agree to transfer advanced missiles to Iran,” Mark Dubowitz, the executive director of the Foundation for the Defense of Democracies, told The Daily Beast at the time. “If Putin became angry enough over the West’s financial punishment of Russia, he could put in play the S-300 deal.”

The Kremin’s decision now sends a signal to Tehran that the sanctions that brought Iran to the negotiating table are done—even before a final nuclear agreement is signed. “Clearly, this is the sanctions regime already starting to crack and fall apart in anticipation there will be an agreement [on nuclear issues with Iran] on June 30,” Conley said. “This is the first major signal that regime is coming to an end.”

From a practical military standpoint, the sale of the S-300 would directly challenge the U.S. position that “all options are on the table” should Iran try to subvert the nuclear deal. The addition of the powerful missile defense system would make punitive airstrikes against Iran extremely difficult.

Many U.S. defense officials from the Air Force, Navy, and Marine Corps agree that the Russian missile system effectively renders entire regions no-go zones for conventional jets like the F-16 or Navy F/A-18 Hornet. Currently, only high-end stealth aircraft like the $2.2 billion B-2 Spirit—of which the Air Force has exactly 20—and the high-performance F-22 Raptor can safely operate inside an area protected by the S-300 and its many variants. The Pentagon’s $400 billion F-35 Joint Strike Fighter will eventually be able to operate inside those zones, too. But according to multiple sources within the Pentagon and defense industry, no warplane now operating can remain inside those well-defended areas for long.

A senior U.S. Marine Corps aviator said that if Russia delivers the S-300 missile to Iran, it would fundamentally change U.S. war plans. “A complete game changer for all fourth-gen aircraft [like the F-15, F-16 and F/A-18]. That thing is a beast and you don’t want to get near it,” he said.

The sale of the S-300 also would neutralize any possibility that Israel could take unilateral action against Iran, one senior Air Force commander noted. The S-300 would effectively prevent the Israeli air force from attacking Iran until the F-35 is delivered to that nation.

“I find it almost hilarious that the Russians are saying, ‘It’s an entirely defensive system and cannot attack anyone, including Israel,’” the senior officer said. “But it also essentially makes Iran attack-proof by Israel and almost any country without fifth-gen [stealth fighter] capabilities. In other words, Iran, with the S-300, can continue to do what they want once those systems are in place without fear of attack from anyone save the U.S. Brilliant chess move…”

But even when Israel receives the F-35, the relatively short-range stealth fighter can only carry a pair of 2,000-pound bombs—which are not likely to be adequate for the most heavily fortified Iranian targets. Some of the Iranian facilities are likely to require the use of the massive 30,000-pound GBU-57A/B Massive Ordnance Penetrator (MOP) that can only be carried by the American B-2 stealth bomber.

An attack on Iran’s nuclear facilities was going to be a daunting task, even under the best of circumstances, another Air Force official with extensive experience flying stealth aircraft said. The targets are deeply buried—which makes them hard to crack open with bombs—and the facilities are scattered all over the place. The Air Force’s tiny fleet of B-2 stealth bombers would have to do most of the work because only those aircraft have the range and weapons needed to hit those targets properly. The introduction of any version of the S-300 would make that extremely difficult job much more challenging, the official said.

But the exact number and exact location of where the S-300s are placed makes a big difference, the official said. That’s further compounded by the fact that the S-300 system is mobile—and can move at a moment’s notice.

If there is a large number of those S-300 air defense systems in place, even pilots flying stealth jets like the B-2 and F-22 Raptor would find the mission to be extremely difficult. “If they’re all over every square inch of the country, then it doesn’t matter what you put out there—it’s going to be a challenge,” the Air Force official said.

But it’s not just Iran. If Russia and China continue to sell advanced air defenses around the world, the overwhelming majority of current U.S. warplanes will be unable to fight in many parts of the globe. “We are very concerned with the proliferation of big [surface-to-air missiles]…now in Crimea, Kaliningrad, and Iran if this is true,” said one senior U.S. Air Force official. “We’re being denied access faster than we can appreciate, in my opinion.”

That said, there are some ways in which older non-stealth jets can fight in areas protected by these new missiles, one senior Air Force official said. But it would be very risky. “It would be really classified to discuss specific SAM [surface-to-air missile] counter tactics, but you know that the ‘double digits SAMs’ [which is what Air Force pilots call the S-300 and its variants] give all of the fourth-gen jets great pause,” the official said.

One way would be to use a combination of miniature air-launched drones carrying jammers to try to spoof the S-300’s radars by giving it false targets, another Air Force official said. Those drones would have to be combined with stealthy long-range missiles to eliminate the Russian-built air defenses.

It would also be very helpful to have a jamming aircraft to try to suppress the S-300’s radar from a distance—which is where the Navy comes in. “[The Navy’s] EA-18Gs [carrier-based jamming aircraft] with their ALQ-218 [electronic sensor] will detect, fix, and track that weapons system,” said a former senior naval aviator. “Once you have it fixed, they can jam and you can employ weapons from range to destroy it.”

But the problem is that the S-300 is a mobile system and thus moves every so often; U.S. pilots can never be sure where a weapon is at any given time. “Well, yes...you can kill it” with the right cruise missile, said Mark Gunzinger, an air power analyst with the Center for Strategic and Budgetary Assessments in Washington. “This assumes, of course, that the S-300 launcher remains at a fixed location after a standoff cruise missile is launched at it.”