U.S. efforts to penetrate reclusive North Korea have been so confounding for so long that the military likely doesn’t have enough accurate intelligence to take out its nuclear and missile facilities even if President Donald Trump ordered it.

Trump on Thursday declared anew that “military action would certainly be an option,” one that would be “a very sad day for North Korea.”

“Is it inevitable? Nothing’s inevitable,” he said at a White House news conference, adding that he “would prefer not going the route of the military, but it’s something certainly that could happen. Our military has never been stronger.”

But because the so called Hermit Kingdom has long been one of the most impenetrable intelligence targets — the top U.S. spy earlier this year called it “one of the hardest, if not the hardest” — there is low confidence airstrikes or other means of attack would successfully thwart its nuclear and missile ambitions without leaving significant elements of its arsenal for Pyongyang to retaliate with.

“You don’t want to stir the hornet’s nest and the hornets are still there when you’re done,” said Douglas Paal, vice president for studies at the Carnegie Endowment for International Peace and a former National Security Council staffer for former President George H.W. Bush. “If you’re giving options to the president … one of the very first things we have to say is we can strike what we can see, but we don’t know what we can’t see.

“Generally speaking, I don’t think it’s overstating to say we’re still groping in the dark,” he added.

The difficulty collecting and interpreting intelligence on North Korea is one reason why different spy agencies often reach different conclusions about North Korea’s capabilities — and have been caught off guard repeatedly, including reportedly by the test earlier this week of a thermonuclear bomb.

In 2013, for example, the Defense Intelligence Agency said it had “moderate confidence” that North Korea had developed a nuclear warhead that could be launched atop a ballistic missile. Shortly after, then-Director of National Intelligence James Clapper walked that back and said that the finding was not the consensus of the whole intelligence community.

But this summer, a military intelligence analysis of North Korea’s nuclear and missile programs had to be revised, concluding Pyongyang could place an atomic bomb atop an intercontinental ballistic missile capable of reaching the United States by the end of 2018 — two years sooner than previous estimates.

Weeks later, other U.S. intelligence agencies concluded that North Korea has begun building missile-ready warheads.

And on Sunday, North Korea’s underground detonation of a nuclear weapon in the remote northeast of the country literally shook the region with what experts now say was a 140 kiloton blast — more powerful than its previous five tests combined.

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