In the early days of his war against Ukraine, President Vladimir Putin told the world that he had ordered his nation’s nuclear forces to a higher state of readiness. Ever since, pundits, generals and politicians have speculated about what would happen if the Russian military used a nuclear weapon.

What would NATO do? Should the United States respond with its own nuclear weapons?

These speculations all sound hollow to me. Unconvincing words without feeling.

In 1958, as a young scientist for the U.S. Navy, I witnessed the detonation of an 8.9-megaton thermonuclear weapon as it sat on a barge in Eniwetok Atoll, in the Marshall Islands. I watched from across the lagoon at the beach on Parry Island, where my group prepared instrumentation to measure the atmospheric radiation. Sixty-three years later, what I saw remains etched in my mind, which is why I’m so alarmed that the use of nuclear weapons can be discussed so cavalierly in 2022.

Although the potential horror of nuclear weapons remains frozen in films from Hiroshima and Nagasaki, the public today has little understanding of the stakes of the Cold War and what might be expected now if the war in Ukraine intentionally or accidentally spins out of control.

The test I witnessed, code-named Oak, was part of a larger series called Hardtack I, which included 35 nuclear detonations over several months in 1958. With world concern about atmospheric testing mounting, the military was eager to test as many different types of weapons as it could before any atmospheric moratorium was announced. The hydrogen bomb used in the Oak test was detonated at 7:30 a.m. A second bomb was set off at noon on nearby Bikini Atoll.

In a nuclear detonation, the thermal and shock effects are the most immediate and are unimaginable. The fission-fusion process that occurs in a thermonuclear explosion happens in a millionth of a second.

As I watched from 20 miles away, all the materials in the bomb, barge and surrounding lagoon water and air, out to a radius of several feet, had been vaporized and raised to a temperature of tens of million degrees.

As the X-rays and neutrons from the bomb raced outward, they left the heavier material particles behind, creating a radiation front that was absorbed by the surrounding air. The radiation, absorption, reradiation and expansion processes continued, cooling the bomb mass within milliseconds.

The outer high-pressure shock region cooled and lost its opacity as it raced toward me, and a hotter inner fireball again appeared.

This point in the process is called breakaway, occurring about three seconds after detonation, when the fireball radius was already nearly 5,500 feet.

By now, the fireball had begun to rise, engulfing more and more atmosphere and sweeping up coral and more lagoon water into an enormous column. The ball of fire eventually reached a radius of 1.65 miles.

Time seemed to have stopped. I had lost my count of the seconds.

The heat was becoming unbearable. Bare spots at my ankles were starting to hurt. The aluminum foil hood I had fashioned for protection was beginning to fail.

I thought that the hair on the back of my head might catch on fire.

The brightness of light the detonation created defies description. I worried that my high-density goggles would fail.

Keeping my eyes closed, I turned until I could see the edge of the fireball.

As I again turned away from the fireball, I opened my eyes inside the goggles and saw outlines of the trees and objects nearby.

The visible light penetrating my goggles increased, and the heat on my back grew more intense. I squirmed to distribute the heat from my side to my back.

About 30 or 40 seconds after detonation, I took off the goggles and watched the angry violet-red and brown cloud from the fireball.

As the rising cloud started to form a mushroom cap, I waited for the shock wave to arrive. In the distance, I could see a long vertical shadow approaching. I instinctively opened my mouth and moved my jaw side to side to equalize pressure difference across my eardrums, closed my eyes and put my hands over my ears.

Pow!

It hit me like a full body slap, knocking me back. I opened my eyes to see another shadow approaching from a slightly different direction. Over the next few seconds, I felt several smaller blows created by reflections of the pressure wave off distant islands.

The fireball kept expanding and climbing at over 200 miles per hour, reaching an altitude of about 2 miles. The boiling mass 20 miles away turned into a mixture of white and gray vapor and continued its climb until it reached somewhere about 100,000 feet.

Meanwhile, the lagoon water had receded like a curtain being pulled back, and the sea bottom slowly appeared. Shark netting that usually protected swimmers lay on the bottom.

Finally, the water stopped receding and appeared to form a wall, like pictures of Moses parting the sea. The wall seemed to remain motionless before finally roaring back.

The water receded for a second time, then repeatedly in smaller and smaller waves and finally as minuscule oscillations across the lagoon surface that lasted all day.

Mankind conducted more than 500 nuclear tests in the atmosphere before moving operations underground, where we tested 1,500 more. Tests to verify the design of weapons. Tests to measure the impact of radiation on people. Tests to make political statements.

During my early Navy career, I focused on scenarios involving nuclear exchanges that could have killed tens of millions of people — what was known during the Cold War as mutually assured destruction.

But the end of the Cold War didn’t bring an end to these fearsome weapons.

Just a few months ago, in January, Russia, China, France, Britain and the United States issued a joint statement affirming that a nuclear war cannot be won and must never be fought.

“We underline our desire to work with all states to create a security environment more conducive to progress on disarmament with the ultimate goal of a world without nuclear weapons with undiminished security for all,” the statement read.

If nuclear weapons are used in Ukraine, the biggest worry is that the conflict could spin quickly out of control. In a strategic war with Russia, hundreds of denotations like the one I witnessed could blanket our countries.

Having witnessed one thermonuclear explosion, I hope that no humans ever have to witness another.

Rod Buntzen is the author of “My Armageddon Experience: A Nuclear Weapons Test Memoir.” He lives in Portland, Ore.