Guam, of course, is the US territory that North Korea threatened August 9 with "an enveloping fire" from its Hwasong-12 "strategic ballistic rocket". The U.S. took back control of Guam on July 21, 1944.
The declaration came after Trump said that the US' nuclear arsenal was "far stronger and more powerful than ever before". The North may have also wanted to demonstrate the reach of its medium-range ballistic missiles, he said.
Japan could legally intercept a North Korean missile headed towards Guam, its defence minister said on Thursday, but experts believe Japan does not now have the capability to do so.
"The market has calmed down a bit, as they remember that the market did not suffer a major damage in April after North Korea filed a missile", said Hiroyuki Nakai, chief strategist at Tokai Tokyo Research Center.
Adding to Trump's remarks, US Defense Secretary Jim Mattis warned Wednesday that Pyongyang must "stand down" in its pursuit of nuclear weapons and refrain from any actions that would result in the demise of its regime.
Still, South Korea - the United States ally with the most to lose from a confrontation with the North - does not appear to be too concerned.
Its proximity to China, Japan, the Philippines and the Korean Peninsula has long made the island an essential position for the us military.
The Korean People's Army (KPA) has disclosed plans to fire four intermediate-range missiles over Japan to land in waters just 18 to 25 miles from Guam.
Beijing's official Global Times cautioned Washington to avoid floating military options, saying such a move makes Pyongyang's threats "not only verbal" as shown in its accelerating nuclear and missile progress.
Experts said the detail provided by North Korea made it likely it would follow through with its plans to avoid being seen as weak or lacking in resolve.
"It's a very serious topic for sure, and it can be scary, but you know, if it's gonna happen, it's gonna happen", she says.
It's created to destroy ballistic missiles during their final phase of flight.
Guam is home to key USA air and naval bases, from which it regularly dispatches strategic assets such as nuclear bombers and nuclear-powered submarines to the peninsula for regular drills or shows of force.
The U.S. military has an enormous presence on the Pacific island.
If any of the four long-range missiles successfully made it through USA defense, Mount said, it would be a huge victory for the rogue state.
Two United States Air Force B-1B bombers, on August 7, conducted a 10-hour sequenced bilateral missions with South Korea and Japan showing that they are ready to "Fight Tonight".
"This is a system that has never been called upon to shoot down a North Korean missile before".