And while Washington hasn't come out and said it, Lewis said the March 15 announcement that the Pentagon will deploy additional ground-based missile interceptors on the West Coast may signal that the North Koreans have deployed a long-range missile they put on display at a parade in 2012. Lewis said the announcement was "mostly for show," but could reflect real U.S. concerns about those missiles.
"If you're going to spend $1 billion to deploy interceptors, they ought to come right out and say it," he said.
But nukes aren't everything
North Korea also has plenty of conventional military firepower, including medium-range ballistic missiles that can carry high explosives for hundreds of miles, as well as thousands of cannons, rocket launchers and tanks massed across the Demilitarized Zone that separates North and South. Seoul is within range of many of those weapons, and the North has threatened before to turn the southern capital into a "sea of fire."
A North Korean bombardment could kill tens of thousands of people in Seoul before South Korean and U.S. retaliation could smash those guns, Hayes said. But that would essentially launch a new Korean War -- one he said would end badly for the long-impoverished North.
"They have less than 30 days of fuel and no ability to refuel," he said. "They've got to fight a very short war before they're just walking to where they're going to fight."
Pyongyang keeps its forces massed on the DMZ "precisely because they're weak," he said.
There are other avenues. When computers at South Korean banks and broadcasters began to crash on Wednesday, suspicion initially fell on the North. South Korea has accused the North of similar hacking attacks before, including incidents in 2010 and 2012 that also targeted banks and media organizations. Adam Segal, a cybersecurity expert with the Council on Foreign Relations, said the hacking is consistent with previous North Korean actions.
So now what?
For years, Pyongyang has made deals to curtail its nuclear and missile work in exchange for economic aid. Those deals have fallen apart when the North went on to conduct other tests. The six-party talks among the North, its Asian neighbors and the United States fizzled in 2007, and the North's first attempt at a satellite launch scotched a previous U.S. plan to trade hundreds of thousands of tons of food for a halt to weapons work.
"I think the problem right now is that you cannot engage them directly after they have done a series of ballistic missile and nuclear tests, and we are going into a period of sanctions now through the U.N. Security Council resolution," Cha said.
"They don't want to give up their nuclear weapons. They want to be able to have their cake and eat it, too. And U.S. policy for the past quarter-century has been these things are all on the table if you are willing to give up your nuclear weapons," he said. "And so this is the problem. This is the dilemma right now."
Meanwhile, the United States is going ahead with joint U.S.-South Korean military exercises amid the North Korean threats, adding a special little twist -- overflights by massive B-52 bombers. It's a move reminiscent of the worst days of the Cold War, and one Hayes called "tactically smart but strategically stupid."
"The North Koreans will have noted it for what it is -- an affirmation of the fact that we're playing the nuclear game with North Korea, and that's the last thing we want to do," he said. "I think our posture is either to persuade ourselves that we're hanging tough, which is a domestic game in Washington, or to reassure our allies and dissuade South Korea from going it alone with nuclear weapons."
But both Hayes and Lewis said there's little to lose by continuing to engage the North.
"We do what we can on defense, and if the North Koreans want to bargain or haggle, I'm prepared to do that," Lewis said.