Three weeks before becoming president, Donald Trump weighed in on the threat of North Korea developing a nuclear warhead capable of reaching the U.S. “It won’t happen,” he vowed on Twitter.

Now planners are contemplating what a U.S. strike to prevent that development might look like, and the options are grim.

Analysts estimate North Korea may now possess between 10 and 25 nuclear weapons, with launch vehicles, air force jets, troops and artillery scattered across the country, hidden in caves and massed along the border with South Korea. That is on top of what the U.S. estimates to be one of the world’s largest chemical weapons stockpiles, a biological weapons research program and an active cyberwarfare capability.

And with Seoul and its 10 million residents just 56 kilometers (35 miles) south of the border — well within North Korea’s artillery range — any eruption of hostilities could have devastating human and economic costs. That is why the North Korean dynasty founded by Kim Il Sung has long hinged its survival, in part, on a warning that any attack could provoke all-out war.

“Unless you were in a crisis situation where we thought the North Koreans were getting ready to attack us, a pre-emptive strike against the North Korean nuclear and missile program is simply not a practical option,” said Gary Samore, a former White House coordinator for weapons of mass destruction, proliferation and terrorism, who is now at Harvard University’s Belfer Center. “This has always been the problem for the U.S. and our allies.”

After Trump ramped up his rhetoric against North Korea this month, the Pentagon ordered the USS Carl Vinson to head toward Korean waters, where the aircraft carrier is expected to arrive next week after some initial confusion in the administration on when it would go. Trump has warned that if China — North Korea’s closest ally — can’t help rein in the regime, the U.S. and its allies will.

Among the war-game scenarios at the Pentagon’s disposal are an airstrike using precision-guided munitions, launched from submarines or stealth aircraft, against the Nyongbyon nuclear reactor facility, where North Korea has produced plutonium for its bombs. That was an option weighed as far back as the Clinton administration, according to two former Pentagon chiefs.

“We were highly confident that it could be destroyed without causing a meltdown that would release radioactivity into the air,” Ash Carter and William Perry wrote in a report for the Belfer Center back in 2002. That plan was seen as a worst-case scenario.

Stealth bombers

Another option would be an attack on facilities at Punggye-ri, the mountainous site in the northeastern part of the country where previous underground nuclear tests have been conducted. The website 38 North, which focuses on North Korea, said satellite images signal recent activity in preparation for another nuclear test. Evading radar, B-2 bombers built by Northrop Grumman Corp. could drop “bunker buster” bombs to try to do the most underground damage.

Or, if the U.S. learned that North Korea was preparing to test an intercontinental ballistic missile — and it had confidence in where that missile would be launched — it could take out the vehicle, or try to shoot it down.

That probably wouldn’t save Seoul from devastation if North Korea responded to such a strike with a barrage of artillery or shorter-range missiles. In its defense, South Korea would go after the artillery that North Korea has massed near the demilitarized zone and use its Patriot missiles and anti-missile ships. In its final months, the Obama administration agreed to deploy a missile defense system known as THAAD to South Korea, but that shield isn’t fully installed yet.

The decision to attack isn’t Trump’s alone. Because South Korea would bear the brunt of any North Korean response, the highest levels of the South Korean military and government would “all have a say in making momentous decisions” like “do we or do we not go to war,” said Bill McKinney, a retired U.S. Army colonel who spent more than 40 years involved in U.S.-Korea military relations and planning.

Any unilateral military action by the U.S. would threaten deep damage to its alliance with Japan, which also would be put at risk, and could bring China and the U.S. into conflict.

‘A crapshoot’

Yet the overarching challenge in an attack on North Korea continues to be gauging the regime’s response. While the U.S. military might want to do something that sends a message but doesn’t start another Korean War, Pyongyang remains strategically unpredictable. Outside analysts have to scour satellite imagery, state-run media, official regime photos and interviews with defectors to glean the barest clues about life and politics in the “hermit kingdom.”

“Our ability to see into North Korea is so curtailed that we don’t have the ability to make well-reasoned judgments about what’s going on,” McKinney, the retired army colonel, said in an interview. The United States’ ability to know what weaponry is even in North Korea and where it is located “is always a bit of a crapshoot,” he said.

North Korea’s unpredictability has only increased under Kim Jong Un, grandson of founder Kim Il Sung, who has had family members and top military aides killed for real or perceived slights. Even a smaller U.S. strike, like the volley of cruise missiles Trump fired at Syria this month, might generate a response that is far from proportional.

Casualty estimates

“Some might say, ‘Look, you know they can’t respond because they’re fearful of the consequences,’ ” said Bruce Klingner, a senior fellow at the Heritage Foundation. But Kim’s regime could also say, ” ‘Well, guys, game on.’ You can’t guarantee what North Korea will do.”

Klingner was the Central Intelligence Agency’s deputy division chief for Korea from 1996-2001, after President Bill Clinton also considered strikes when North Korea was found to have been developing a nuclear capability. He said contingency scenarios at the time included estimates of hundreds of thousands of casualties, “and that was before they had nuclear weapons.”

The situation facing the U.S. grows more dire as North Korea moves toward its goal of developing an intercontinental ballistic missile with a nuclear warhead that could hit the U.S. mainland. But in weighing possible responses, the administration must also decide how urgent that threat really is.

North Korea will need until at least 2020 to develop a nuclear weapon with that reach, according to John Schilling, a satellite specialist with the Aerospace Corp. The country still hasn’t tested an ICBM, though it does have about 1,000 ballistic missiles, Schilling said.

No ‘imminent crisis’

“This isn’t an imminent crisis,” Schilling told reporters Tuesday in a briefing organized by 38 North. “The imminent threat is to South Korea and Japan.”

But Schilling referred to the regime’s unpredictability, saying, “Probably their first response will not be nuclear — it might not even involve missiles,” Schilling said. “They have several levels of escalation to go before they get to nuclear weapons.”

In taking on North Korea so directly, Trump confronts a problem that bedeviled his predecessors from both political parties. Six-nation talks, direct bilateral negotiations, food aid and United Nations sanctions have all failed to deter the Kim dynasty. Even China, Pyongyang’s ally, has been snubbed by the Kim regime repeatedly over the years.

The debate over possible U.S. military responses predates anyone in the federal government today. President Richard Nixon considered tactical nuclear strikes after North Korea shot down a U.S. reconnaissance plane in 1969, according to documents declassified in 2010 and published by the National Security Archive.

War games

“Nixon and his advisers were forced to heed the Pentagon’s warnings that anything short of massive attacks on North Korea’s military power would risk igniting a wider conflagration on the peninsula, leaving diplomacy, with all its frustrations, as the remaining option,” Robert Wampler at the National Security Archive wrote at the time.

More than 30 years after Nixon, The Atlantic magazine ran a war-gaming scenario, assembling experts to come up with what a U.S. response would look like. Run by retired U.S. Air Force Col. Sam Gardiner, the exercise ultimately ended in discord, with little agreement aside from the consensus that the North Korean problem would only grow worse over time.

Gardiner says events now have born that out. The Trump administration, like those before it, appears to have no clear objective for North Korea, Gardiner says, whether it is regime change, preventing Pyongyang from getting a nuclear weapon or something else. And he says most options for action could result in all-out war or, short of that, spur the regime to perfect the nuclear weapons it so desperately wants — and which Trump says he won’t let it get.

“In essence, there is no military option,” Gardiner said. Asked what plan of action he would present to Trump if forced to pick one, he responded, “I would resign first.”