PYONGYANG, North Korea >> Early one winter morning, North Korean leader Kim Jong Un stood at an observation post overlooking a valley of rice paddies near the Chinese border. Minutes later, four projectiles plunged into the sea off the Japanese coast.

North Korea had just run its first simulation of a nuclear attack on a U.S. military base.

North Korea, which is testing ballistic missiles faster than ever, is rapidly becoming a better equipped and more formidable adversary. Some experts believe it might be able to build missiles advanced enough to reach the United States in two to three years.

And that poses a game-changing problem for the U.S., which for its part is also escalating, successfully shooting down a target ICBM launched from a Pacific island with a California-based interceptor missile on Tuesday.

If North Korea launches a pre-emptive nuclear strike on an American military base in Asia, would the U.S. recoil and retreat? Would it strike back, and risk losing Washington in a second wave of attacks?

In the March launch, North Korea sent four Scuds into the ocean 300 to 350 kilometers (185 to 220 miles) off Japan’s coast. State media called it a drill of troops who will “strike the bases of the U.S. imperialist aggressor forces in Japan in a contingency.” They said Kim was accompanied at the launch by nuclear weapons specialists.

Analyst Jeffrey Lewis and his colleagues at the Center for Nonproliferation Studies in Monterey, California, quickly realized the Scuds were on a trajectory that, with a southerly tweak, would have sent them raining onto Marine Corps Air Station, Iwakuni, on the southern tip of Japan’s main island.

Before the simulation, U.S. and South Korean forces were conducting joint military drills involving F-35 fighters based at Iwakuni, home to some 10,000 U.S. and Japanese personnel. The F-35s had reportedly trained for a “decapitation strike” on Kim Jong Un and his top lieutenants.

Kim, apparently, was practicing how to take them out first.

The Cold War concept of “mutually assured destruction” works when each side is convinced neither would survive. North Korea isn’t likely to reach that stalemate level. If it succeeds in building nuclear-tipped ICBMs that can reach the U.S. mainland, the dynamic could be much more volatile.

Bruce Bennett, a North Korea expert and defense analyst with the RAND Corporation, offers these scenarios:

— North Korea has a stockpile of nuclear warheads and the ability to launch them from submarines or remote, hard-to-detect sites on land. Fearing an attack from the U.S., it launches a pre-emptive nuclear strike on the South Korean port of Busan and tells the United States that if there is any nuclear retaliation, it will fire nuclear weapons at U.S. cities.

Would President Donald Trump, or whoever succeeds him, risk losing Los Angeles, or Chicago, to defend America’s allies?

— North Korea tries another ballistic missile launch like the one on March 6. But just before the missiles hit the water near Japan, a nuclear weapon on one or more of them detonates, downing a few commercial aircraft or sinking some cargo ships. This would convince the world that Kim Jong Un has a real nuclear arsenal and isn’t shy about using it.

Would Trump react with a nuclear attack on North Korea?

— War breaks out on the Korean Peninsula. North Korea launches an ICBM that appears to be coming down short, well west of California. But on the way down it bursts in a nuclear explosion, possibly causing some damage to U.S. territory. Pyongyang then threatens more serious damage to the United States if there is any nuclear retaliation or U.S. intervention in the conflict, raising the risk of millions of people dead.

“With the weight of history on his shoulders, how would a U.S. president respond?” Bennett asks. “How should he respond?”

On April 15, Kim Jong Un watched military units from his million-man armed forces goose-step by, and then applauded at the most varied array of missiles and transport vehicles the North has ever displayed.

The message of this year’s military parade was clear. North Korea is, or is nearly, able to strike pre-emptively against a regional target. It is preparing to withstand a retaliatory attack if it does, and it is building the arsenal it needs to then launch a second wave of strikes, this time at the U.S. mainland.

Its vision of a new “balance of terror” built to a crescendo as six submarine-launched missiles and their land-based cousin rumbled through the square.

Submarines are the ultimate stealth weapon, mobile and hard to find, and the land-launched missile version was also all about stealth. It uses solid fuel, which means it can be stored, hidden and moved to rough terrain for a quick launch. Kim Jong Un has ordered it be mass produced.

The big reveal of the parade came next: the unveiling of the “Hwasong 12.”

A month after the parade, it was sent 2,111 kilometers (about 1,240 miles) in altitude and remained airborne for 30 minutes. The North’s media said it can carry a “large-size heavy nuclear warhead.”

Kim claimed it shows he has an “all-powerful means for retaliatory strike.”

That’s bravado. The missile’s estimated striking range is 4,500 kilometers (2,800 miles), give or take.

But, put another way, it’s halfway to Chicago.