On Friday morning, a North Korean ballistic missile flew over the Japanese island of Hokkaido and into the Pacific Ocean. This was the second time in less than three weeks that a missile has flown over Japan; worryingly, it came just a day after Pyongyang said that Japan “should be sunken into the sea.”

However, as sirens sounded in Japan, Americans quickly grew concerned that the real threat revealed by this latest missile test was not just against Japan but also the U.S. territory of Guam.

North Korea's missile, believed to be an intermediate-range Hwasong-12, had flown 2,300 miles in just over 17 minutes, according to South Korea's Joint Chiefs of Staff. That range means that Guam, which lies 2,100 miles from North Korea, is now in reach.

“North Korea demonstrated that it could reach Guam with this missile,” David Wright, co-director of the global security program at the Union of Concerned Scientists, wrote in a blog post.

As tensions between North Korea and the United States and its allies have increased in recent weeks, Pyongyang has made a number of threats against Guam, an island territory of 160,000 people. In early August, a North Korean military spokesman said that the country was considering a plan to fire missiles into the sea around Guam. A propaganda video released a few weeks later reiterated this suggestion.

It's an ominous thought. Guam might have only 14 minutes to react if a North Korean missile were about to strike, officials have said.

Guam has faced similar warnings from North Korea in the past. The island territory, which is around 4,000 miles west of Hawaii, is an important strategic hub for U.S. power in the Pacific. It is home to both Andersen Air Force Base and Naval Base Guam, bases that contain not only 6,000 troops but also long-range bombers, ships and submarines.

“Every time there is some saber rattling in this part of the world, Guam is always part of the occasion,” Robert A. Underwood, president of the University of Guam and a former delegate to the House of Representatives, told The Washington Post in August.

Here's why North Korea threatened an island in the Pacific that's home to 160,000 people. (Victoria Walker, Danielle Kunitz/The Washington Post)

However, rapid advancements in North Korea's missile technology have changed the nature of the threat. Notably, missile tests earlier this year were fired at a lofted trajectory — essentially sending the missile high into space to avoid flying it over other nations. Two Hwasong-14 intercontinental ballistic missiles tested in July appeared to be capable of reaching the U.S. mainland, but it remains unclear how they would fare on a standard trajectory.

With the past two tests that apparently used Hwasong-12 missiles, Pyongyang appears to have moved away from that cautious strategy and has conducted tests over Japanese territory — a tactic that involves considerable risk.

The increase in range between the Aug. 28 and Sept. 15 launches would appear to show that Guam is in range of the Hwasong-12 — in theory, at least. However, there are still a number of unknown variables in the test, including the size of its payload and how a heavier payload would affect its flight. In his blog post, Wright adds that it is also unlikely that the missile is accurate enough to hit a military base on Guam.

“Even assuming the missile carried a 150 kiloton warhead, which may be the yield of North Korea’s recent nuclear test, a missile of this inaccuracy would still have well under a 10 percent chance of destroying the air base,” Wright wrote.

But North Korea is still likely to proclaim the latest missile test a success. After the Aug. 28 test, North Korean leader Kim Jong Un said it was “a meaningful prelude to containing Guam,” the state-run Korean Central News Agency reported.

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