The false alert was a stark reminder of what happens when the old realities of the nuclear age collide with the speed — and the potential for error — inherent in the internet age. The alert came at one of the worst possible moments — when tension with North Korea has been at one of the highest points in decades, and when Mr. Kim’s government has promised more missile tests and threatened an atmospheric nuclear test.

During the Cold War there were many false alarms. William J. Perry, the defense secretary during the Clinton administration, recalled in his memoir, “My Journey at the Nuclear Brink,” a moment in 1979 when, as an under secretary of defense, he was awakened by a watch officer who reported that his computer system was showing 200 intercontinental ballistic missiles headed to the United States. “For one heart-stopping second I thought my worst nuclear nightmare had come true,” Mr. Perry wrote.

It turned out that a training tape had been mistakenly inserted into an early-warning system computer. No one woke up the president. But Mr. Perry went on to speculate what might have happened if such a warning had come “during the Cuban Missile Crisis or a Mideast war?”

The United States faces an especially difficult problem today, not just because of tense relations with North Korea but also because of growing fears inside the military about the cyber vulnerability of the nuclear warning system and nuclear control systems.

Because of its location, Hawaii — more than any other part of the United States — has been threatened by escalating tensions and the risks of war, and preparations have already begun there.

On Friday, the day before the erroneous alert, several hundred people attended an event in Honolulu sponsored by the Chamber of Commerce in which military commanders, politicians and others discussed the threat to the islands’ population.

“The U.S. is the designated recipient — and that’s because we are public enemy No. 1 to North Korea,” Dan Leaf, a retired Air Force lieutenant general and Pacific Command deputy commander, was quoted as saying in The Honolulu Star Advertiser.