Today it might feel like we’re on the brink of nuclear war.

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The President of the United States is calling Kim Jong Un, the unstable leader of an emerging nuclear power, "Rocket Man."

And that isolated leader is making not-so-vague threats:

“ It's not a mere threat but a reality that I have a nuclear button on the desk in my office," Kim said. "All of the mainland United States is within the range of our nuclear strike.”

It feels scary. But imagine living in 1949 and reading this statement from President Truman: "We have evidence that within recent weeks an atomic explosion occurred in the U.S.S.R."

Up until that moment, the United States believed it was the world’s only nuclear power.

It used that power to level the cities of Hiroshima and Nagasaki in Japan, bringing an end to World War II.

The hope was that the atomic bomb would keep the United States safe. Now the Soviet Union had one too. The Soviet test kicked off the Cold War race to build more powerful, destructive weapons.

It was a sobering wake-up call for U.S. cities. If a nuclear bomb could be dropped on Japan, it could happen in the U.S. The country had to get prepared, and Portland led the way.

Portland: The Preparedness Poster Child

After the Soviets launched the atomic bomb test heard around the world, the U.S. reacted with establishment of the Federal Civil Defense Administration. The goal was to prepare the public to handle a nuclear threat through training, media and eventually money for cities.

Portland seized the opportunity. Voters there kicked in an additional $600,000 for the city’s civil defense program.

Related: Why A Small Texas Town Wants Oregon’s Nuclear Waste

The crown-jewel was the construction of a state-of-the art underground bunker and civil command center. Built into Kelly Butte, the center had enough space for about 250 city officials, radio communication capabilities and supplies to be fully operational for seven days.

On Sept. 27, 1955, the city showed off its shiny new civil defense program with Operation Green Light.

The goal: Evacuate 1,000 city blocks as fast as possible. Portland's answer was green lights. When the sirens sounded, thousands of people left work and homes downtown. Cars followed a new traffic pattern set up by Civil Defense officials with green lights lining the exit routes.

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In less than an hour, more than 100,000 people evacuated. The Oregon Historical Society calls it the largest evacuation in the nation at that time.

The drill was so successful it inspired CBS film crews to recreate the evacuation in "The Day Called 'X'," a fictional documentary using Portland as the location.

Portland was a poster child for civil preparedness. And just eight years later, they were the first city in the country to disaffiliate with the national civil defense program.

Two reasons explain why.

The Birth Of The Hydrogen Bomb And Death Of Portland Civil Defense

On Columbus Day in 1962, a deadly storm tore through the Northwest. Wind gusts hit 144 mph and ripped down more than 15 billion board feet of timber. The violent natural disaster claimed 46 lives.

Portland’s civil defense program — which received that $600,000 levy for emergencies just like this — was, according to The Oregonian, "strangely silent." In fact, the agency wasn’t even asked by the city to act at all.

Disillusioned voters didn't forget it.

The second reason why the influential program ended was because nuclear weapons got a lot more powerful.

During those eight years, intercontinental ballistic missile technology improved. It shot Sputnik 1 into orbit.

Related: The Waste That Remains From Arming Nuclear Weapons

More dramatically, atomic bombs were being replaced by hydrogen bombs.

That's a big difference. If the atomic bomb used on Hiroshima were instead dropped on the center of Portland, the heaviest impact would be felt over just a few square miles of city.

A hydrogen bomb like the Soviets tested, on the other hand, would cause destruction from the Coast Range clear to Mount Hood if dropped in the same place.

Portland City Commissioner Stanley W. Earl started calling the civil defense program a hoax. He said it gave citizens a false sense of security and went as far to claim "(public shelters) would become crematoriums."

Portland closed its civil defense program in 1963.

Today it seems naive to believe that Operation Green Light could protect Portlanders from a nuclear bomb.

Looking back, though, the drill was remarkable. The evacuation of 1,000 city blocks was the result of more than 100,000 people rallying behind a single mission.

The idea of preparation has evolved with new human and environmental threats. But coming together to overcome whatever the future may hold could be the only sense of security we can ever have.