Russian President Vladimir Putin used an annual speech this week to make bold claims about “invincible” new Russian weapons that could easily overcome U.S. missile defenses. Among the weapons Putin said had been successfully tested was a cruise missile with a nuclear-propulsion engine — a weapon that would fly at a low altitude and follow an unpredictable flight path to avoid U.S. missile defense systems.

Speaking to Megyn Kelly of NBC News after the speech, Putin said that “every single weapons system discussed today easily surpasses and avoids an antimissile defense system.”

But how worried should Americans be about Russia's new missiles?

It's certainly true that, if Putin's claims are accurate, they represent a step forward in missile technology that existing U.S. missile systems would be unable to match. There's another important detail here, however: U.S. missile defense systems are almost certainly no match for existing Russian missiles, either.

Kingston Reif, director for disarmament and threat reduction policy at the Arms Control Association, said that Russia would have “no problem whatsoever” overcoming U.S. missile defenses with its existing weapons. As such, Putin's shiny new weapons don't necessarily change the core strategic balance with the United States.

They may, however, show the limits of trying to contain nuclear threats from nations such as Russia or even smaller threats like North Korea by missile defense alone.

Why Russian missiles are hard to hit, even now

Building a missile that can accurately deliver a nuclear weapon to the other side of the world is difficult — it is rocket science, after all. But building another weapon that can take out that missile before it strikes its target? That's even harder.

The idea of a missile defense system to protect the United States first appeared during the Cold War. In the 1980s, President Ronald Reagan pushed a Strategic Defense Initiative, colloquially known as Star Wars, that would seek to offer complete protection to the United States from ballistic missiles. However, the system was criticized for being unrealistic and too costly.

After the collapse of the Soviet Union, this all-encompassing idea of missile defense was reshaped to focus on protecting the United States from smaller “rogue” states such as North Korea and Iran. As The Washington Post reported Friday, the Trump administration is now planning a change away from this, to a missile defense system that could deal with resurgent threats from Russia and China.

The United States has 44 missile interceptors based in Alaska and California, as well as sea and land-based systems in Europe and Asia. Though there are plans for a modest increase in these numbers, experts say they offer little protection against a well-equipped country such as Russia, which had sophisticated technology as well as sheer numbers that could overwhelm U.S. systems.

“They do not come close to even putting a dent in Russia’s ability to threaten the U.S. homeland with nuclear weapons,” Ian Williams, an expert on missile defense technology with the Center for Strategic and International Studies, wrote in an email. “Just the numbers alone ... 44 homeland defense interceptors vs. 1,500 Russian nuclear warheads.”

Indeed, there are reasonable doubts about the current systems' ability to block a threat from a relatively unsophisticated missile power such as North Korea. Last year, Pyongyang conducted a number of missile tests at night — a worrying idea, as U.S. missile systems in Alaska and California have had only limited and unsuccessful testing in the dark.

North Korea has also made technological advancements that may allow it to launch multiple missiles at once from mobile launchers, a tactic that could put a big strain on missile defense technology if used in a real-world scenario.

The Post’s Paul Sonne explains the current state of the U.S. missile defense system. (Joyce Lee, Paul Sonne/The Washington Post)

How Russia's new weapons are different

The big difference between the missiles Russia already has and the ones announced Thursday is that existing missiles are ballistic, whereas the new missiles would be cruise missiles.

Ballistic missiles are launched into the atmosphere and then come down toward their target, whereas cruise missiles fly low to the ground. The key development outlined in Putin's speech was that Russia's cruise missile would be nuclear-powered, which would significantly increase their range and allow them to make lengthy detours to avoid detection.

Given the differences in the way the missiles fly, the defensive systems designed to deal with them need to be different.

“As a rule of thumb, ballistic missiles are easy to see, hard to hit. Cruise missiles are the opposite, easier to hit but hard to see,” Williams explained. “This is because they fly low, hug the terrain, follow the curve of the Earth. By the time your ground-based radar picks them up, they are on top of you.”

While the United States does have systems designed to counter ballistic missiles, they would not work against a cruise missile. And though there are interceptors designed to destroy cruise missiles, it can be hard to see them coming — U.S. radar systems designed for ballistic missiles wouldn't catch cruise missiles in time, as they cannot see over the curve of Earth.

This doesn't mean they are impossible to defend against. Williams suggested that some kind of elevated radar system, on an aircraft or balloon or even in space, would be able to spot cruise missiles. A new U.S. missile defense policy may also emphasize the ability to take out a missile before it is launched. The United States does not currently have these capabilities, and developing them will be difficult and costly.

However, the most compelling deterrence against launching a nuclear strike — the threat of a retaliatory nuclear strike — is little affected by Russia's new missile technology.

Why Putin's announcement is worrisome

Russia analysts say that Moscow views U.S. missile defense as a threat because of concerns that the United States might strike first and take out most of Russia's arsenal, then rely on its missile defense system to stave off a Russian retaliatory strike. However, if Putin was really concerned about this, he wouldn't need a nuclear-powered cruise missile.

“They could have just added more missiles or penetration aids” if they were really worried about missile defense, said Jeffrey Lewis, a nuclear expert at the Middlebury Institute of International Studies. “It takes something more to push the Russians to go past a modest numerical expansion and embrace these kinds of crazy sci-fi weapons.”

Putin's decision to publicly announce the new weapons technology may be largely domestic in focus. Russia is just weeks away from a presidential election. Though Putin is expected to win easily, most analysts think that the Kremlin is hoping for high turnout — something that a little patriotic nuclear-missile unveiling may help with.

But the decision to unveil new weapons this week is probably not only about the election. Russia appears to be falling back on Soviet patterns of decision-making, Lewis said, as democratic institutions fail. “In this case, there is a powerful military industrial complex that, like during the Soviet years, gets a lot of leeway to explore this kind of crazy stuff,” he added.

The United States, in its proposed expansion of both its nuclear arsenal and missile defense systems, could wind up playing into a tit-for-tat series of escalations with Russia, which had already undermined the bilateral arms control agenda by violating the 1987 Intermediate-Range Nuclear Forces (INF) Treaty.

“Neither side is taking into account the concerns the other has about their offensive and defensive developments sufficiently seriously to avoid increased risks of instability,” Reif said. “Neither side is putting forth arms control proposals to reverse the current trajectory and mitigate risks to stability.”

To arms control proponents, Russia's new weapons are far from a game-changer, but they should serve as a reminder that missile defense alone is no substitute for a shared understanding with Russia and other nuclear countries about the risks of nuclear war.

“Americans have never been comfortable accepting the idea of vulnerability,” Lewis said. " These weapons don’t change our vulnerability, although they do force us to confront it.”

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