WASHINGTON — At the start of Donald Trump’s presidency, US intelligence agencies told the new administration that while North Korea had built the bomb, there was still ample time — upward of four years — to slow or stop its development of a missile capable of hitting a US city with a nuclear warhead.

The North’s young leader, Kim Jong-un, faced a range of troubles, they assured the new administration, giving Trump time to explore negotiations or pursue countermeasures. One official who participated in the early policy reviews said estimates suggested Kim would be unable to strike the continental United States until 2020, perhaps even 2022.

Kim tested eight intermediate-range missiles in 2016, but seven blew up on the pad or shattered in flight — which some officials attributed partly to a US sabotage program accelerated by President Barack Obama.

And while the North had carried out five underground atomic tests, the intelligence community estimated that it remained years away from developing a more powerful type of weapon known as a hydrogen bomb.

Within months, those comforting assessments looked wildly out of date.

At a speed that caught US intelligence officials off guard, Kim rolled out new missile technology and in quick succession demonstrated ranges that could reach Guam, then the West Coast, then Washington.

And on the first Sunday in September, he detonated a sixth nuclear bomb. After early hesitation among analysts, a consensus has emerged that it was the North’s first successful test of a hydrogen weapon, with explosive force some 15 times greater than the atomic bomb that leveled Hiroshima.

The CIA and other US intelligence services had predicted this moment would come, eventually. For decades, they accurately projected the broad trajectory of North Korea’s nuclear program.

Yet their inability to foresee the North’s rapid strides over the past several months now ranks among the United States’ most significant intelligence failures, current and former officials said in recent interviews.

That disconnect — they saw it coming, but got the timing wrong — helps explain the confusion, mixed signals, and alarm that have defined how Trump’s untested national security team has responded to the nuclear crisis.

In an interview, Lieutenant General H.R. McMaster, Trump’s national security adviser, acknowledged that Kim’s race to the finish line “has been quicker and the timeline is a lot more compressed than most people believed.’’

As a result, he argued, “We have to do everything we are doing with a greater degree of urgency, and we have to accelerate our own efforts to resolve the issue short of conflict.’’

Senior intelligence officials said they began investing more heavily in acquiring information on North Korea’s weapons program in 2012, reaping benefits over the past two years. But they acknowledged they made two key assumptions that proved wrong.

They assumed that North Korea would need about as much time to solve the rocket science as other nations did during the Cold War, underestimating its access to both advanced computer modeling and foreign expertise.

They also misjudged Kim, 33, who took control of the dynastic regime in late 2011 and made the weapons program more of a priority than his father or grandfather did.

Obama warned Trump during the transition a year ago that North Korea would pose the most urgent national security threat, and almost immediately the newly installed president began repeating, publicly and privately, that he inherited “a mess’’ in North Korea because his predecessors did not do enough.

Former officials in the Obama administration dispute that. But some concede that the intelligence community’s flawed assessment of the North’s progress meant there was less pressure to bolster missile defenses, more vigorously enforce sanctions, or consider stepped-up covert action.

It is not clear that even with more advanced warning the Obama or Trump administrations would have been able to slow Kim’s progress.

The shakiness of intelligence on North Korea casts a shadow over Trump’s options going forward.

Many in the Pentagon see the failure to anticipate the North’s recent breakthroughs as an ominous reminder of how much could go wrong. A successful preemptive strike, for example, might require precise knowledge of the locations of manufacturing facilities, nuclear plants, and storage areas, and confidence that cyberstrikes and electronic strikes would cripple Kim’s ability to retaliate.

Trump, however, was not disturbed by the absence of warning, McMaster said. “He understands human nature and understands he will never have perfect intelligence about capabilities and intentions.’’

Ever since the United States began tracking North Korea’s efforts to obtain a nuclear weapon, a pattern has repeated itself: US intelligence agencies excelled at forecasting the direction and overall timeline of the program, yet repeatedly missed critical turns.

From as early as 2000, the National Intelligence Council was remarkably prescient about North Korea’s overall direction, predicting in an unclassified report that it would “most likely’’ have a nuclear missile that could hit US cities by 2015.

When the United States was mired in the first year of the Iraq War, the council refined its prediction, saying a “crisis over North Korea is likely to come to a head sometime over the next 15 years,’’ that is, no later than 2019.

None of this was ignored. President George W. Bush began a program to interdict ships delivering material for the North’s weapons program, and he accelerated secret efforts to cripple the program by sabotaging its supply chain with bad parts.

But the CIA’s main focus was on counterterrorism, and satellite coverage over North Korea was often diverted to keep troops safe in the Middle East.