WASHINGTON (AP) — Just when President Donald Trump's strategy for North Korea was finally starting to show dividends, he threw it a curve.

At long last, his administration seemed to be speaking with one voice on a key national security issue, a surprisingly elusive task in Trump's first six months. But he upended all that with a threat to slam the North with "fire and fury like the world has never seen" if it provoked America again.

By inflaming the situation, Trump also may have undermined the only serious prospect for resolving the North Korea crisis: successful cooperation with China.

Trump's strategy has relied on a delicate diplomatic two-step: increasing pressure on China in hopes that, in turn, China will use its influence to get North Korea to abandon its nuclear aspirations. Trump had been deeply frustrated by China's recalcitrance, but there were signs it was finally coming around, including its vote in favor of the toughest U.N. sanctions in a generation — possibly ever.

On its face, Trump's move seemed to fit a pattern in which he becomes his own biggest obstacle to achieving his objectives. Sometimes he exposes divisions within his administration that others can then exploit. Other times, Trump stakes out positions so unpopular that needed partners can't afford to work with him. In other cases, he's played directly into his critics' worst suspicions about him.

That tendency has been on display repeatedly, including in his firing of the former FBI director who had been investigating his campaign's possible Russia collusion, his "Muslim ban" comments that sunk his travel ban in court, and siding with Saudi Arabia over Qatar in the Persian Gulf crisis just as his administration was trying to mediate.

With North Korea, it was the budding prospect for the cooperation he'd been seeking from Beijing that was jeopardized by his latest unforeseen move.

In recent months, Trump had been so frustrated by Beijing's reluctance to help that he let it be known he was considering new, punitive trade actions. The Treasury Department had even prepared unprecedented "secondary sanctions" targeting Chinese companies and banks that deal with North Korea.

But Trump agreed to hold off after China, with its U.N. Security Council vote, signaled it was finally moving in the direction Trump wanted, U.S. officials said.

So when Trump seemed late Tuesday to lower himself to Kim Jong Un's level, aggressively threatening the North with physical force, it wasn't just Washington that was taken aback.

"For the leader of the most powerful country in the world to be talking about the ability to annihilate another country with a power never seen before not only gets a reaction from Pyongyang, but from Beijing, and Europe," said Jeffrey Bader, formerly President Barack Obama's top Asia hand who now runs the Brooking Institution's China program. "That is not helpful to our interests."

Even before his threat, U.S. officials had been concerned that China would only halfheartedly enforce the sanctions. So Trump's administration put Beijing on notice it would be closely monitoring its compliance. Secretary of State Rex Tillerson was in Asia pressing other North Korean neighbors to be strict, too. He also offered Pyongyang a way out: Halt missile tests, and eventually, the U.S. would return to negotiations.

But in the wake of Trump's threat, it's far from clear that China — the North's economic lifeline — will feel compelled to punish Pyongyang economically, a step that China ultimately feels is against its interests. The sanctions aim to cut off one-third of the North exports, starving it of cash for its nuclear and ballistic weapons programs.

China may see Trump's apocalyptic missive as evidence he isn't truly interested in the kind of diplomatic solution that would preserve Beijing's No. 1 interest: Stability in its neighborhood. Without Beijing's cooperation, the sanctions would have negligible effect.

So what drove Trump to upset the balance on North Korea so dramatically? And why now?

One possible trigger was a news report about a new U.S. military assessment claiming the North had mastered the ability to fit a nuclear warhead atop one of its long-range missiles — a key step toward being able to strike the U.S. Another may have been Pyongyang's threat of "thousand-fold" revenge for the new U.N. sanctions.

Trump's threat blindsided Tillerson while he was in Asia, according to officials familiar with the timeline. And while his alliterative "fire and fury" admonition seemed premeditated, Trump aides insisted that while they'd discussed with the president what tone he'd strike if asked about North Korea, they didn't know exactly how he'd phrase it. The aides weren't authorized to comment publicly and spoke on condition of anonymity.

North Korea's military soon issued its own menacing warning about potential plans to attack Guam, and alarm spread. Tillerson, heading home from Asia, jumped on the phone with Trump for roughly an hour, before working to calm the situation by telling reporters on his plane that Americans should "sleep tight" knowing the situation hadn't changed and there wasn't "any imminent threat."

But Trump soon inflamed the situation again, with tweets boasting of a U.S. nuclear arsenal he said has no equal. Across the government, officials rushed to try to show Trump wasn't improvising national security on the fly. Defense Secretary Jim Mattis warned the North against "actions that would lead to the end of its regime and the destruction of its people." And State Department spokeswoman Heather Nauert insisted "we are all singing from the same hymn book."

"We've had a lot of messages," said former U.N. Ambassador Bill Richardson, who's traveled to North Korea several times. "We've had a diplomatic message from the secretary of state, the national security adviser, the U.N. ambassador, they seem to have different messages, more aggressive. Pre-emptive military strikes. We need to cool down and be rational because this is a very grave situation."

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Associated Press writer Jonathan Lemire and AP Washington Bureau Chief Julie Pace contributed to this report.

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Josh Lederman has covered foreign policy, national security and the White House for The Associated Press since 2012. Reach him on Twitter at http://twitter.com/joshledermanAP