North Korean leader Kim Jong Un inspecting the long-range strategic ballistic rocket Hwasong-12. Thomson Reuters

After strong words from President Donald Trump promising "fire and fury" in response to continued threats from Pyongyang, North Korea threatened to launch a nuclear-missile attack on the US military in Guam.

But North Korea mentioned a specific missile, the Hwasong-12, that it has tested just once, and an expert contacted by Business Insider says hitting the US military in Guam would be easier said than done.

"No one, not even the North Koreans, knows the CEP of the HS-12," Mike Elleman, the senior fellow for missile defense at the International Institute for Strategic Studies, told Business Insider, referring to circular error probable, or the distance by which a missile can be expected to miss its target.

"Having said that, we can guesstimate a minimum CEP from first principles," Elleman said. Essentially, even a small task like cutting off the engines a fraction of a second too late can make the missile go miles off course.

"All told, the CEP will be greater than 5 km," Elleman said. (Five kilometers is equal to about 3 miles.) "But this is a very rough estimate. Given the paucity of flight tests, I suspect if used today, the HS-12 would have a CEP considerably larger than 5 km, perhaps 10 km, or more."

While Andersen Air Force Base, the home of the strategic bombers North Korea expressly wanted to target, spans about 35 miles across, North Korea would have greater problems than accuracy.

The massive blast radius caused by a nuclear device, which North Korea, according to news reports this week, is believed to have perfected and miniaturized to deliver on missiles, could make up for a lack of accuracy. But serious questions remain around North Korea's ability to build vehicles to reenter the planet's atmosphere through tremendous pressure and friction. The only test of the Hwasong-12 was done on a lofted trajectory and not a realistic, flatter curve.

Four B-1B Lancers assigned to the 9th Expeditionary Bomb Squadron, deployed from Dyess Air Force Base in Texas, arriving at Andersen AFB in Guam on February 6. US Air Force

"The more interesting and important question revolves around the missile's reliability," Elleman said. Though Elleman thinks the Hwasong-12 would work more often than not, North Korea would need to launch several missiles, and there's just no evidence it has enough deployed and available.

Additionally, the US military at Guam has the Terminal High Altitude Area Defense missile-interceptor system, perhaps the most reliable anywhere.

"We always maintain a high state of readiness and have the capabilities to counter any threat, to include those from North Korea," Lt. Col. Christopher Logan, a Pentagon spokesman, told Business Insider.

When North Korea tested its intercontinental Hwasong-14 missile on July 4, the US was aware 70 minutes ahead of time, according to the Diplomat. After North Korea issues a warning like the one it did Tuesday, the US would be even more on guard and may intervene if it spotted North Korea preparing a launch.