Last week, CIA director Mike Pompeo said North Korea could only be from having the ability to hit the United States with a nuclear-armed intercontinental ballistic missile. Speaking at the , Pompeo said America needs to behave with the knowledge that North Korea is “on the cusp” of being able to strike the U.S. with a nuclear weapon. All of which betrays a simple fact: North Korea has become a nuclear power.

This is something the Trump administration must accept, despite promises that a nuclear armed North will not be allowed to exist. It has been 21 years since Pyongyang conducted its first nuclear test.

Five more have followed, along with dozens of missile tests, including the first flight of an ICBM this past July. Once the North successfully weaponizes an ICBM, it will be one of only three nations, including Russia and China, able to target the U.S.

The U.S.’s traditional strategy of deterrence has provided security for the U.S. throughout the Cold War and the post-Cold War era, but despite that success, many in the current administration are looking beyond deterrence to deal with North Korea. There is, of course, an American missile defense system that has been operational since 2004 and has received more than $40 billion in funding. But a former head of the middle defense agency said the system has chance of destroying a warhead before it could detonate above an American city. That, in other words, is far from full proof.

More worryingly, though, is that the long-simmering crisis between North Korea and the U.S. has accelerated this year, with North Korea conducting its sixth underground nuclear test and launching 22 missiles during 15 launch events. Combined with the war of words flowing back and forth between Pyongyang and Washington, the tensions between the two nations is greater than it has been in decades. Former CIA director John Brennan said last week that while the chances of war are not likely, he did of a military confrontation occurring as high as 25 percent.

At the , national security advisor Gen. H. R. McMaster admitted there was still time to solve the crisis diplomatically, though that window of time is running out. As North Korea marches toward an operational, nuclear-armed ICBM, McMaster reiterated President Trump’s position on the matter. “[Trump] is not going to accept this regime threatening the United States with nuclear weapons. There are those who would say, well, why not accept and deter. Well, accept and deter is unacceptable,” McMaster said.

Accept and deter may not work for the Trump Administration, but accept and deter has worked for several generations for the United States, Russia, and China who have been pointing ICBMs at one another since the 1950s. Last year, its latest ICBM design, the Agni V, which is not designed to further deter Pakistan, but is a signal to Beijing of New Delhi’s growing military strength—and their level of strategic deterrence.

Strategic deterrence has also prevented the use of a nuclear weapon since August 9, 1945, when Nagasaki was destroyed. And despite decades of intense rivalry and Cold War pressure, the U.S. and the Soviet Union did not clash in some World War III simply due to possession of massive arsenals of nuclear weapons. Many agree that a similar strategy of deterrence can work with a nuclear-armed North Korea, including Susan Rice, who was the national security advisor for President Obama.

In an August op-ed for , Rice wrote “But war is not necessary to achieve prevention, despite what some in the Trump administration seem to have concluded. History shows that we can, if we must, tolerate nuclear weapons in North Korea—the same way we tolerated the far greater threat of thousands of Soviet nuclear weapons during the Cold War.”

Indeed, the Trump administration does seem quite determined to dismiss the idea of deterrence when it comes to North Korea. McMaster, shortly after Rice’s op-ed disagreed with her assessment of the strength of deterrence when applied to North Korea. “No, she’s not right,” McMaster said. “And I think the reason she’s not right is that the classical deterrence theory, how does that apply to a regime like the regime in North Korea? A regime that engages in unspeakable brutality against its own people? A regime that poses a continuous threat to the its neighbors in the region and now may pose a threat, direct threat, to the United States with weapons of mass destruction? A regime that imprisons and murders anyone who seems to oppose that regime, including members of his own family, using sarin nerve gas in a public airport?”

McMaster, who graduated from West Point and has a , is no idiot, but you would think he would have a better appreciation of Soviet history, given that, in fact, they have posed a much greater threat to the U.S. than North Korea does. Joseph Stalin was also responsible for more deaths than Hitler, with nearly 20 million Russians perishing in “” under his rule.

The in a report that hundreds of thousands have perished in North Korean prisons reserved exclusively for Pyongyang’s political prisoners. Death sentences are for theft of grain and for possessing media produced in South Korea. The government of Kim Jung Un, and those before him, have ruled North Korea with an iron fist, caring little for its citizens.

The North has repeatedly claimed that its true objective was to bring about the reunification of the Korean peninsula by absorbing the South, however, the main objective for Pyongyang has always been survival; indeed, the swift demises of Saddam Hussein and Muammar Gaddafi have also provided some clear motivation. Neither of them had nuclear weapons, and should they have had those weapons, the North believes, both men would still be in power today.

Why Deterrence Must Work

Earlier this month, President Trump Sean Hannity, “We have missiles that can knock out a missile in the air 97 percent of the time, and if you send two of them, it’s going to get knocked down.” He was talking about missile defense and the ability of America to defend itself should North Korea decide to send an ICBM hurtling toward San Francisco or some other American city. But Trump’s estimate of the system’s abilities is severely overstated, an overconfidence that suggests he will pursue preventive measures over deterrence.

Which would be a mistake, since the most effective means of stopping an ICBM is to destroy the weapon before it is even launched. Once in the air, the ICBM has a strong advantage over any current missile defense system deployed today, including the Ground-based Midcourse Defense System, the American system. Straightforward in its approach, ballistic missile defense is still an expensive and complicated endeavor filled with technical challenges that have frustrated scientists since President Reagan first suggested shooting down ICBMs in 1983.

Our Missile Defense System Is Questionable

Several hours north of Anchorage is Fort Greeley, one of the main bases for the GMD system. It is here that most of the Ground-based Interceptors are housed, waiting in silos to be launched to destroy an incoming nuclear warhead. By the end of 2017, Fort Greeley will be the home for 40 GDIs, with 4 interceptors also being located at Vandenberg Air Force Base in California. The GBIs are the heart of the GMD system, designed to intercept a warhead outside of the Earth’s atmosphere at the weapon’s highest trajectory. They are the bullet that will hit another bullet.

Or at least they will try.

The GMD system was declared operational in 2004, pushed through by the second Bush administration in the shadows of 9/11 and the “Axis of Evil.” But GMD has struggled to successfully engage targets during a series of test engagements that stretch back to 1999. In 18 tests, only ten have been successful. Five tests have been conducted since 2010 with only two successes, including the most recent test in May, when an interceptor successfully brought down an ICBM-class target for the first time.

The GMD system is designed to intercept a nuclear warhead during the second of three phases of flight for an ICBM. The flight path of an ICBM includes an initial “boost” phase, when it launches from the surface into space; a “midcourse” phase, when the warhead separates from the rocket and travels in space above the earth’s atmosphere; and a “terminal” phase, when the warhead streaks back down through the Earth’s atmosphere, descending to its target.

Built around a network of sensors, radars, satellites and GBIs that stretch across , the missile defense system is, in other words, a complicated one that has thus far yielded poor results—failures that may actually embolden North Korea to rapidly grow their ICBM numbers.

That’s because the American missile defense system was never designed to protect against a swarm of ICBMs, and by launching more than one missile, the system could be quickly overwhelmed. It has been estimated that as many as four or five interceptors may have to be launched to achieve one kill.

With only 44 available interceptors, it would not take too many warheads to exhaust the supply. One thing that could make things even harder for the system: if North Korea were to use some means of deception to hide the warhead from American sensors within the so-called “threat cloud,” which might consist of things like metallic chaff and Mylar balloons to confuse radars and sensors.

So far, though, the missile defense system has never had to sort its way through anything like that; the system, indeed, is still struggling with far more basic functions. One organization, the Union of Concerned Scientists, , “As it exists today, the system would offer little to no protection in any realistic scenario. It’s also diplomatically counterproductive, and potentially dangerous; policy makers, misled to believe in missile defense’s effectiveness, may act in ways that increase the likelihood of conflict.”

Thus far, North Korea has used its burgeoning nuclear capabilities in much the same way the U.S. and the Soviet Union did, by attempting to coerce behavior out of other actors, achieving goals essentially through nuclear blackmail. North Korea has had nuclear weapons for over 20 years and still has yet to detonate one over Seoul or Tokyo.

Which is not to say Pyongyang has behaved responsibly toward its neighbors, attacking islands with artillery and sinking a naval warship with a submarine in recent years. It is these small miscalculations that make nearby nations nervous. But the Cold War, too, was not without its periods of heightened tensions.

And, indeed, deterring North Korea is nothing new for the United States. America has been deterring Pyongyang since 1953, when the Korean War was stopped. It must be continued.