On September 30, Russian lawmakers unanimously approved President Vladimir Putin's plan to begin combat operations in Syria—and hours later Moscow's warplanes in the region began attacking what the Russians said were ISIS militants.

Right before the bombs rained down, a Russian general arrived in Baghdad warned the U.S. military planners to keep America's own warplanes out of the way. U.S. officials said they would not alter their flight plans.

This is the beginning of a dangerous new phase of the international intervention in the Syrian civil war. Not only has Russia tried to order U.S. forces to step aside, it actually has the firepower to back up its demands. Some of the 35 warplanes Russia has deployed to Syria are specifically designed for fighting foes like the United States, not ISIS.

Seemingly out of nowhere on September 21, they appeared at an air base in Latakia, a regime stronghold in western Syria—28 of the Russian air force’s best warplanes, including four Su-30 fighters and a number of Su-25 attack planes and Su-24 bombers.

Soon six more Su-34 bombers and at least one Il-20 spy plane followed, part of a contingent of Russia forces reportedly including some 500 troops plus armored vehicles and SA-15 and SA-22 surface-to-air missiles.

For U.S. and allied officials observing the deployment, there has been plenty of cause for confusion…and alarm. It’s not just that, more than four years into Syria’s bloody civil war, Russia has decided to jump in and make things more complicated.

No, it’s what kinds of weapons—planes and missiles, especially—Moscow decided to send, and what those weapons say about the Kremlin’s ultimate plan in Syria. Many of them don’t seem to be well-suited to fighting ISIS. They’re built to battle adversaries like the United States.

To be clear, 35 warplanes and a few surface-to-air missiles aren’t a lot in the grand scheme of things. There’s no shortage of military aircraft flying over Syria five years into the country’s bloody civil war.

Every day some of Syria’s aging Soviet-made planes—from the 300 or so that have survived four years of combat—take off from regime airfields to bomb ISIS militants and secular rebels slowly advancing on Syria’s main population centers.

Meanwhile hundreds of jets from the American-led international coalition have been waging, since the fall of 2014, an intensive air campaign against ISIS and al Qaeda targeting just the militants.

What’s weird and alarming about the Russian contingent is that it’s not really optimal for attacking lightly armed insurgent fighters. Surface-to-air missiles are only good for destroying enemy aircraft, which Syrian rebels do not possess. And the Su-30s are best suited for tangling with other high-tech forces.

Who in region possesses these high-tech forces? The United States, for one. Israel, too. Why, the United States, of course. Russia’s warplanes and missiles in Syria could pose a threat to America’s own aircraft flying over the country—all in order to carve out and preserve a portion of Syria that the United States can’t touch.

Officially, Russia has deployed its forces to Syria to reinforce embattled Syrian President Bashar al-Assad and help defeat the self-proclaimed Islamic State.

“There is no other way to settle the Syrian conflict other than by strengthening the existing legitimate government agencies, support them in their fight against terrorism,” President Vladimir Putin said in an interview with American news networks ahead of his September 28 meeting with President Obama at the United Nations in New York City.

“There are more than 2,000 militants in Syria from the former Soviet Union,” Putin said. “Instead of waiting for them to return home we should help President al-Assad fight them there, in Syria.”

Sure enough, Su-25s, Su-24s, and Su-34s are capable ground-attack planes, roughly equivalent to U.S. Air Force A-10 attack jets and F-15E fighter-bombers.

But that’s only a portion of the Russian air arsenal. The problem is, the Su-30s are next to useless for fighting ISIS. The Sukhoi fighters are primarily air-to-air fighters—and some of the best in the world. Besides Russia, China also flies versions of the twin-engine, supersonic Su-30 and has even begun outfitting them with new air-to-air missiles that U.S. Air Force Gen. Herbert Carlisle has repeatedly described as one of his biggest worries.

In a series of aerial war games in the last decade, India’s own Su-30s have tangled with—and reportedly defeated—American and British fighters in mock combat, sparking minor controversies in both countries as their respective air forces scrambled to explain why the Russian-made planes weren’t necessarily superior to U.S. F-15s and British Typhoon jets.

It’s obvious why Russia, China, and India, among other countries, would deploy Su-30s to counter heavily armed enemies possessing high-tech fighters of their own. But that doesn’t explain the Russian Su-30s in Syria. “I have not seen [ISIS] flying any airplanes that require sophisticated air-to-air capabilities,” U.S. Air Force Gen. Philip Breedlove, the military head of NATO, told an audience in Washington, D.C., on Sept. 28.

Moreover, Breedlove said Russia didn’t need to deploy the SA-15 and SA-22 surface-to-air missiles to Syria if its mission is to help Assad beat ISIS. “I have not seen ISIL flying any airplanes that require SA-15s or SA-22s,” he said, using one of several acronyms for the militant group.

Breedlove said he suspects Russia is trying to set up what the military calls a “anti-access, area-denial,” or A2AD, zone in western Syria. Moscow has recently established these zones in the Baltic region and in the Crimean Peninsula, which Russia seized from Ukraine in 2014. “We are a little worried about another A2AD bubble being created in the eastern Mediterranean,” Breedlove said.

The point of these zones is to give Russia exclusive access to strategic regions, Breedlove claimed. In the case of western Syria, an A2AD zone helps to ensure that Moscow can send forces into the eastern Mediterranean, which NATO has dominated since the Soviet Union’s collapse in 1991.

Russian access to the Mediterranean via Syria requires that Assad’s regime survives, however. In that sense, Moscow’s strategic aims dovetail with the Syrian regime’s goals. Thus the Su-25s, Su-24s, and Su-34s very well could end up joining Damascus’s air war on the rebels and militants. The Su-30s, however, will probably be guarding against a very different enemy.

Of course, high-end warplanes can be repurposed to fight lower-tech foes—the U.S. has done just that, in its decade and a half bombing Afghanistan and Iraq. And many militaries deploy air-to-air fighters merely as precautions. A small contingent of U.S. Air Force F-22 stealth fighters, which can carry bombs but are best at aerial fights, plays a leading role in the coalition air campaign targeting ISIS.

The F-22s act as “quarterbacks,” according to Carlisle, using their sophisticated sensors to spot targets for other planes and also protecting those planes against Syrian fighters and missiles. To date, the Syrian regime has not attempted to interfere with the U.S.-led bombing runs, but the F-22s keep flying.

But neither has the coalition tried to interfere with the Syrian air force’s attacks on opposition fighters—yet. U.S. Army Special Forces have been training, at great expense, a small number of Syria rebels the Pentagon had hoped could form the core of a reinvigorated, secular rebel force that can knock back ISIS.

The problem is, many rebel trainees in the American program have made it clear they prefer to fight the regime first. Many have dropped out of the program in the face of Washington’s demands, compelling the Pentagon to remove them from the training effort. U.S. Defense Secretary Ashton Carter told Congress, using the administration’s preferred acronym for ISIS, that he wants recruits “to have the right mindset and ideology, not be aligned with groups like ISIL...[and] to fight ISIL.”

“It turns out to be very hard to identify people who meet both of those criteria,” Carter added.

Worse, once the recruits complete their training and go to fight ISIS, the U.S. military will have “some obligations” to protect them, Carter said. If U.S.-trained rebels turn their weapons against the Syrian regime and Russian warplanes bomb them, would that compel American F-22s to attack the Russians—and then force the Russian Su-30s to intervene?

It’s not hard to see how Russia’s support of Assad could run afoul of America’s support for secular Syrian rebels—and how Moscow’s effort to establish an aerial foothold in Syria could draw U.S. and Russian jet fighters into battle with each other.

Don’t pretend for a moment that that terrifying notion hasn’t crossed the minds of generals and politicians in both Moscow and Washington.