On the day of North Korea’s first atomic test in 2006, aides to President George W. Bush began phoning foreign capitals to reassure allies startled by Pyongyang’s surprising feat. The test, aides said, had been mostly a failure: a botched, 1-kiloton cry for attention from a regime that had no warheads or reliable delivery systems and would never be allowed to obtain them.

“The current course that they are on is unacceptable,” State Department spokesman Sean McCormack said publicly at the time, “and the international community is going to act.”

A decade later, that confidence has all but evaporated. After a week in which Pyongyang successfully lobbed four intermediate-range missiles into the Sea of Japan, U.S. officials are no longer seeing North Korea’s weapons tests as amateurish, attention-grabbing provocations. Instead, they are viewed as evidence of a rapidly growing threat — and one that increasingly defies solution.

Over the past year, technological advances in North Korea’s nuclear and missile programs have dramatically raised the stakes in the years-long standoff between the United States and the reclusive communist regime, according to current and former U.S. officials and ­Korea experts. Pyongyang’s growing arsenal has rattled key U.S. allies and spurred efforts by all sides to develop new first-strike capabilities, increasing the risk that a simple mistake could trigger a devastating regional war, the analysts said.