North Korea’s second intercontinental ballistic missile test in a month’s time is leading to stepped-up defenses, offensive shows of force and nuclear-disaster planning across the Pacific — changing the dynamic in the process for American cities that until recently thought they were outside the rogue nation’s reach.

Based on early information, the Union of Concerned Scientists said Friday’s ICBM test shows that North Korea could easily reach the West Coast and a number of major cities.

The missile was launched on a very high “lofted” trajectory, allowing it to come down in the Sea of Japan, rather than overflying Japan. If flown on a standard trajectory, the missile could have a range of 6,500 miles, not taking into account the rotation of the earth, and much more than that if considering the degree of spin, the organization said.

Hawaii is 4,660 miles from North Korea.

Hawaii’s potential vulnerability prompted the state to become the first to undertake disaster planning for the remote possibility of a North Korean missile strike. Other population centers are now following suit.

“We have spoken directly with California, Guam, Washington (state) and Alaska,” Toby Clairmont, executive officer of the Hawaii Emergency Management Agency, said in an email. “California and Guam stated they are working towards a plan of their own. We have agreed to share our work with them. We have already shared some preliminary info with California.”

On July 21 Hawaii started the rollout of a new plan to prepare its 1.4 million citizens for the unlikely event of a 15-kiloton North Korean nuclear detonation over Honolulu. That’s about the same size as the atom bomb dropped on Hiroshima in 1945.

With just 20 minutes from launch to impact, the guidelines emphasize finding shelter immediately and planning with family where to go and what to do. Hawaii Emergency Management’s website is starting to provide information.

A wavering siren sound used during the air raid warning days of World War II, and not heard since the Cold War, will be reintroduced in testing starting in November.

The North Korean threat level has somewhat bizarrely ushered back 1950s-era practices that would be almost pointless with the megaton yields and multiple warheads of today’s Russian and Chinese nuclear weapons.

American intelligence agencies, meanwhile, have been forced to admit that North Korea’s missile program is more advanced than they thought. That’s something else Hawaii is thinking about.

Clairmont said the state’s planning is predicated on North Korea’s capabilities growing over time, and will be phased and scaled to the assessed level of the threat.

A 15-kiloton detonation at 1,000 feet above ground level is the planning model, with the threat level calculated to be extremely low — for now.

“If North Korea is able to develop a megaton-range weapon and place it on a ICBM — verified by (U.S. Pacific Command and others) — then the model needs to change,” Clairmont said. “A megaton-range weapon could dramatically change our level of effort. We see that as many years ahead, however, (and) the preparedness steps we are currently taking are still the right thing to do now.”

Across the Pacific, Japan is looking at incorporating one or more Aegis Ashore missile defense sites on its shores similar to one on Kauai that’s used for testing only. South Korea is moving ahead with more Terminal High Altitude Area Defense, or THAAD, missiles.

In response to North Korea’s Friday launch, the U.S. and South Korean armies test-fired offensive missiles into South Korean territorial waters in a demonstration “countering North Korea’s missile launch/nuclear test,” the 8th U.S. Army said.

The show of force used the U.S. Army Tactical Missile System and South Korea’s Hyunmoo Missile II.

“The ATACMS can be rapidly deployed and engaged and provides deep-strike precision capability, enabling the (South Korean)-­U.S. alliance to engage a full array of time-critical targets under all weather conditions,” 8th Army said in a release.

After North Korea first flight-tested an ICBM on July 3, the United States and South Korea conducted an initial barrage of ATACMS and Hyunmoo Missile II test-firing along the east coast of South Korea.

On July 7 two Air Force B-1B Lancer bombers from Andersen Air Force Base on Guam conducted a 10-hour flight mission with South Korean and Japanese fighter jets in response to North Korea’s escalating actions. Additional force demonstrations are expected following Friday’s ICBM test.

Michael Elleman, formerly a Lockheed Martin research and development scientist who focused on solid propellants, weapons elimination technology and nuclear effects, said on the website 38North.org that the two ICBM tests show the still-evolving nature of the North Korean threat.

In the July 3 test, the second stage of the Hwasong-14 missile was undersize and underpowered, making it ill-suited for use on a ballistic missile, he said. While Friday’s missile remained to be confirmed, the data were “fully consistent with a Hwasong-14 tested with a larger second stage that is powered by a high-thrust engine.”

“If the above assessment is correct, North Korea seems to have made a logical step forward as it tries to perfect the technologies to build and field an operationally viable ICBM that can threaten the mainland United States,” Elleman said.