On Monday, Britain’s army chief explained how Russia sees its air defense systems as the cornerstone of its evolving military doctrine.

And on Tuesday, Russia hinted it might be planning to use its most advanced air defense system, the S-400, to push U.S. forces out of Syria. The evidence for this possibility came via Kremlin propaganda outlet, RT, which declared that Russia has deployed four new S-400 air defense missile systems to Syria. This adds to an existing portfolio of S-400s that the Russians already have operating in Syria.

But why is this latest arrival so newsworthy? Because Russia’s bench of S-400 units now allows it both to defend its military bases on Syria’s Mediterranean coast and to create an air defense bubble around any advancing forces elsewhere in the country. Put simply, Russia does not have much money and does not have the need to deploy more S-400s to Syria unless it's for more offensive-oriented missions.

These new S-400s provide specific potential in two key areas.

First, they could provide air cover for any Turkish military effort that pushes east towards Manbij, the Euphrates river and Kurdish-held but U.S. "safe-zoned" territory. While Turkey is currently fighting to push U.S. supported Kurdish rebels out of an enclave in northwestern Syria, it has thus far held back from advancing on the Euphrates buffer. Still, if Turkish President Recep Tayyip Erdogan did decide to push east (which, considering his grandiose ego and ambition, cannot be ruled out), the S-400 could give the Turks air cover with which to deter any U.S. fighter-bomber aircraft that sought to challenge their offensive. Erdogan knows as much and is spending big bucks to buy the S-400 from Russian President Vladimir Putin.

Fortunately, the possibility of an overt Turkish assault under Russian air cover is unlikely in that it would risk a showdown with the U.S. that Turkey’s economy and NATO membership could not afford.

The second and much likelier S-400 deployment would be alongside Syrian forces operating just south of the Euphrates river valley in eastern Syria. Assad and Russia claim those forces are focused on fighting the Islamic State, but their actual priority is pushing the U.S. out of another coalition safe zone area north of the Euphrates. From Putin’s perspective, getting the U.S. gone will allow him to secure Russia's long term interests without bothersome distractions. As I’ve explained, Russia is actively threatening U.S. ground forces in its pursuit of a U.S. exodus.

And while the Trump administration is rightly standing firm against Putin’s saber rattling, the arrival of a more potent S-400 capability would pose new threats to U.S. air crews guarding the safe zone. Consider that where the U.S. presently scares off Russian air force fighters by sending up F-22s to greet them, any forward deployment of the S-400 would create a layered threat challenge for U.S. air crews to manage.

It would also give Putin added means to threaten U.S. aircraft to stay around 100 kilometers north of the Euphrates river. After all, if we are to put any stock in Russian claims that the S-400 can target F-22s up to 150 miles away, then we should assume 100 km is roughly the limit of their range (the Russians like to exaggerate). Even assuming the F-22s would be very hard for the Russians to target, the new S-400 threat assessment would force the U.S. to send up electronic warfare aircraft to support its combat fighters by jamming Russian radar and targeting systems.

This isn't just about military match-ups, however.

The ground-based S-400 also gives Putin a different opportunity in that it appears more defensive in nature than, say, a SU-25 or SU-35 fighter jet. As an exceptionally skilled manipulator of Western public opinion, Putin might believe that the S-400 gives him a way to pressure the U.S. out of Syria without appearing to be the aggressor.

He might, for example, say something like: "This air defense bubble is for the protection of our forces and allies and is wholly defensive. But if coalition jets enter our defense zone and pose a threat to our ground forces, we will defend ourselves. Therefore, they must stay 100 kilometers away."

Facing such a statement, President Trump and Defense Secretary Jim Mattis would have to choose between ordering U.S. air crews to penetrate the bubble and risk being fired on, or to stay 100 km away. Crucially also, even if the U.S. continued to fly in any bubble, would its close allies do the same? And would the U.S. gradually decide the risks of Putin's slow rolling escalation were too high? How might these calculations affect U.S. diplomacy with regards to negotiations over Syria's political future?

Putin thinks about all these things.

Nevertheless, the U.S. must not back down. In response to any S-400 deployment towards the front lines, the U.S. should make it clear that we’re equipping our fighters with air to ground missiles and will defend against any engagement. If Putin wants to turn on his air "defense" bubble, that’s fine; we can ignore it.

But if he wants to try and enforce a bubble, we must be ready to let him eat S-400 wreckage.