On Tuesday, North Korea threatened a ballistic missile attack on the U.S. territory of Guam.

It must not be considered an idle threat. While some analysts assume North Korea does not have effective missile reentry vehicles, I assume they do. And located 2,100 miles from North Korean launch sites, Guam is well within range of Kim Jong Un's increasingly advanced ballistic missile forces.

Correspondingly, the island's governor is wrong to pledge, "Currently, there is no threat to our island or the Marianas."

That said, a sense of proportion is necessary here. After all, Guam is well protected.

For a start, North Korean ballistic missile forces are closely monitored by U.S. intelligence capabilities; from satellite imagery, U-2 spy plane and drone surveillance, signals intelligence (listening to orders between headquarters and missile commanders), and other means. In turn, if the U.S. government believed North Korea was preparing to launch a ballistic missile attack on Guam, it would attack North Korean missile forces, launch sites, and probably also command and control facilities. These strikes would be quick, aggressive, and possibly even nuclear.

But let's say North Korea was able to launch a missile, or several missiles, successfully.

In that scenario, U.S. missile defense forces in Japan would kick into overdrive. As the map below indicates, a ballistic missile attack on Guam would have to travel across Japan.

Yet, Japan is protected by the Aegis ballistic missile defense system, designed to intercept ballistic missiles in midcourse (between boost/launch and terminal stages). And as the map shows, any ballistic missile targeting Guam would be midcourse while over Japan.

Even if the Aegis system failed, however, Guam would still not be defenseless.

Via forces at Anderson Air Force base, Guam is protected by the Terminal High-Altitude Area Defense system and, presumably, also Patriot PAC-3 interceptor batteries. These are terminal phase interceptor forces designed to shoot down a ballistic missile as it re-enters the atmosphere in the final stage of an attack.

Finally, there's the size issue. At around 28 miles long and 8 miles wide, Guam is not a big target. That produces an added risk calculation to North Korean strategy. After all, while the range and nuclear warhead-compatibility of North Korean ballistic missiles are impressive, their targeting capabilities are less competent. And were Kim Jong Un to attack Guam but fail to hit his target, he would have sacrificed his regime simply to fire a missile into the Pacific Ocean.

"Sacrifice his regime" are the operative words here. At least with Trump, North Korea knows that a first strike on U.S. territory would result in regime change.