In this April 15, 2017, file photo, an unidentified missile that analysts believe could be the North Korean Hwasong 12 is paraded in Kim Il Sung Square in Pyongyang, North Korea.

In this April 15, 2017, file photo, an unidentified missile that analysts believe could be the North Korean Hwasong 12 is paraded in Kim Il Sung Square in Pyongyang, North Korea. Associated Press / Wong Maye-E

Let's Walk This Through: If North Korea Launches An ICBM, Then...

It takes a lot of rosy assumptions to get to President Trump’s 97% chance of success.

How good is America’s homeland ballistic-missile defense? If a war broke out tomorrow, could it stop an attack from North Korea?

The short answer, despite many assurances from Defense Department officials, is that no one knows. Ballistic-missile defense, or BMD, is a stunningly ambitious and complex undertaking, unforgiving of the smallest problems. An attacker has many built-in advantages, and it is only because of North Korea’s supposed technological backwardness—a doubtful, increasingly out-of-date notion—that the existing defensive system has enjoyed any credence at all.

Still, North Korea’s force of Hwasong-14 intercontinental ballistic missiles, or ICBMs, is a work in progress. In American terminology, it appears to be at a stage called “initial operational capability”—short of full-scale readiness, but available to some extent on an emergency basis.

Trying to anticipate how an exchange would play out feels like writing a script for a new remake of Mothra vs. Godzilla. But let’s give it a whirl.

First, what does the American system look like today? It’s composed of a network of radars, space-based sensors, battle-management systems, and “hit-to-kill” interceptor missiles designed to smash an attacking warhead through the sheer force of the collision.  A total of 36 interceptors are currently deployed—four at Vandenberg Air Force Base in California, and 32 at Fort Greely in Alaska. Another eight are to be installed by the end of the year in silos at Fort Greely. Called the Ground-based Midcourse Defense, or GMD, the system is operated by U.S. Northern Command, which is charged with the defense of American homeland.

The Pentagon’s authority for testing and evaluation rates the system as having “limited capability to defend the U.S. Homeland from small numbers of simple intermediate-range or intercontinental ballistic missile threats launched from North Korea or Iran.” But the evaluators decline to provide “quantitative” assessments of its performance, citing a lack of ground testing of key subsystems with “accredited” models and simulations.

The evaluators also criticized GMD for the low “reliability and availability” of its interceptors, noting that the U.S. Missile Defense Agency “continues to discover new failure modes during testing.” According to MDA, 18 GMD intercept tests were undertaken between October 1999 and May 2017, with 10 hits, although one of these hits may not have “killed” the target. Depending on how one scores that event, the system has an overall success rate of about 50 to 55 percent. Well-informed critics have knocked even this disappointing record as misleading, pointing to the “scripted” nature of testing.

Related: If North Korea Fires an ICBM, the US Might Have to Shoot It Down Over Russia

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Statements from Pentagon officials imply that four interceptors would be fired at each attacking warhead in order to improve overall chances of success. This suggests that President Trump’s recent claim of 97% effectiveness is based on an assumption of a 60% chance of success per interceptor: a 40% chance of failure, multiplied by itself four times to represent four interceptors, declines to a failure rate of less than 3%.

But as James Acton of the Carnegie Endowment for International Peace points out, this calculation optimistically suggests that interceptor failures would come from various random malfunctions, and not from some common problem that could disable all four interceptors. Some difficulty that would affect all interceptors at the same time—let’s say, operating at night, during a period of unplanned maintenance at one or more key radars, or during a period of intense solar activity—could not be overcome by taking multiple shots.

GMD might even be vulnerable to being knocked out of action. Pentagon evaluators point to “known survivability issues,” implying that system may be too fragile to withstand attack, or perhaps even extreme environmental conditions. They add that few assessments of the system’s “cybersecurity” have been conducted.

Finally, the evaluators note “shortfalls” in “radar availability” that affect GMD’s “suitability.” Reporting by the Los Angeles Times suggests that the culprit may be the Sea-Based X-Band radar, better known as SBX: a powerful floating sensor meant to “discriminate” between attacking warheads and decoys. SBX suffers from maintenance problems and has shown poor performance in testing. But perhaps worst of all, it has only a 25-degree “soda-straw” field of view. That means it can only look in one direction at a time, whereas attacking missiles can be directed at widely dispersed targets. These limitations may have contributed to later decisions to deploy different X-band radars in Japan, but they are not ideally positioned to provide discrimination capability as attacking warheads approach North America.

How does that stack up against the threat? First, decoys may now be a real problem. The first ICBM displayed in North Korea—the Hwasong-13, better known as the KN-08—featured a warhead-bearing “reentry vehicle” that was mounted directly on top of the missile, leaving no space for additional payload. But this missile, never flight-tested, seems to have been replaced by the Hwasong-14 (also known as the KN-14), which flew twice in July. Videos and photographs from North Korea show that the Hwasong-14 carries a single reentry vehicle inside a detachable “shroud,” which may offer room for inflatable decoys, chaff, or other “countermeasures” to complicate the problem of spotting the warhead.

The biggest limitation of the Hwasong-14 force so far appears to be a shortage of highly mobile launch vehicles. In a series of parades since 2012, the North Koreans have shown off what appear to be just half a dozen large, modern transporter-erector-launchers, or TELs, built on chassis imported from China in 2011 for “forestry.” In the April 2017 parade, four of these TELs were shown converted to a new configuration, each with a launch canister for some as-yet-unseen missile type. At least one TEL remains in the original configuration, based on photographs of the two July flight-tests.

At the same parade, the North Koreans also showed off four locally made tractor-trailers with launch canisters. Similar launchers could exist for the Hwasong-14, but because the country has only two major paved highways, it could be hard to take them out of hiding for very long during wartime before they can be found and destroyed. Until North Korea can find or build highly capable TELs itself, it will have no more than a handful that can move quickly on back-country roads and into wooded areas, giving them a reasonable chance to emerge from hiding and launch their missiles.

The resulting mental picture is something like a pair of punch-drunk boxers, each one barely able to stand. Whoever prevails might be a matter of blind luck.