Leo Szilard didn’t drop the atomic bomb. He didn’t even work at Los Alamos building atomic bombs. Yet nuclear power and nuclear weapons still depend upon the principles he discovered nearly a century ago.

Szilard forever altered the future, but his personal life was something of a paradox. He conceived the most destructive force ever unleashed by humanity, then changed his mind and worked tirelessly to prevent it from ever being used.

Understanding Szilard’s personal dilemma helps clarify the contradiction that still exists at the heart of nuclear power and nuclear weapons today.

The Idea That Changed the World

Leo Szilard was a Hungarian scientist who spent much of his life living out of a suitcase. Born in Budapest in 1898, Szilard’s ideas include the electron microscope, the cyclotron, the linear accelerator, and the concept of a bit of information – a critical step toward modern computers and the internet we surf today.

But Leo’s grandest vision occurred to him as he strolled the streets of London on a summer afternoon in 1933. The morning newspaper had quoted Ernest Rutherford – one of the most respected scientists of the time – as saying that atomic energy was impossible. A student of Rutherford’s, James Chadwick, had discovered the neutron the previous year, but no one imagined what his discovery might mean.

Except Leo Szilard.

It seemed to Leo that a nuclear chain reaction might occur if an element existed with atoms that ejected more than one neutron when struck by a neutron. Could those ejected neutrons strike the nucleus of other atoms and cause them to eject more than one neutron as well?

That simple idea fundamentally changed the world we live in, and Leo immediately understood the implications of that. But nuclear chain reactions were rejected as fantasy by his peers at the time.

Then on March 12, 1934, Szilard filed British patent number 630,736 – Transformation of Chemical Elements. That’s something worth repeating. Eleven years before the bomb, Leo Szilard patented the process by which nuclear power and nuclear weapons depend to this day. But Leo understood more than just the science. The humanitarian in him clearly saw the terrible destruction his discovery might bring.

From Fantasy to Reality

As Hitler rose to power, Szilard grew deeply concerned that Nazi Germany might build the bomb first. He also believed that England would not provide sanctuary from the Nazis for long, so he immigrated to America in 1938. 

The following year, Leo wrote the most famous letter in nuclear history to President Franklin D. Roosevelt, then convinced his friend Albert Einstein to cosign it. The letter explained the Nazi nuclear threat, and urged Roosevelt to begin nuclear weapons research immediately.

Roosevelt acted. The Manhattan Project was born.

A Scientist with a Conscience

Szilard’s concern with the Nazis was genuine, but he despised what followed. He initially worked with fellow physicist Enrico Fermi to build and patent the world’s first nuclear reactor so the Manhattan Project could make plutonium. But he grew increasingly alarmed by the secrecy involved – and the likelihood that atomic bombs he helped build would soon be dropped on cities filled with innocent children.

In 1945, Szilard formally opposed dropping an atomic bomb on Japan without first exploring less lethal alternatives. He drafted the Szilard Petition, urging President Truman to consider the moral implications of mass-targeting civilians.

Dozens of fellow scientists signed the petition, but no one knows how effective it might have been, since Japan was bombed before it reached Truman’s desk. Devastated and unable to avoid feeling guilty, Szilard turned away from weapons work for the rest of his life.

A Man of Peace

After the war, Szilard became an outspoken critic of nuclear weapons and a passionate advocate for nuclear arms control. He believed that the intelligence used to split atoms must now be used to control the consequences of that split.

Leo foresaw the coming nuclear arms race and called for increasing transparency between nations. In the early days of the Cold War, he even proposed a direct communications channel between Washington and Moscow—an idea that became the famous “red telephone” hotline.

Always busy, Szilard helped establish organizations like The Council for a Liveable World, but still found time for scientific work of a different kind. In an amazing display of versatility, Leo completely switched gears and made significant contributions to the less-lethal fields of molecular biology and genetics.

Yet despite his profound impact and lasting legacy, Leo Szilard remains largely unknown outside scientific and policy circles. The government did not appreciate a former nuclear scientist speaking out against nuclear weapons. Pressure came to bear, and Leo’s name seldom appears in books with presidents, generals, and famous Los Alamos scientists like Robert Oppenheimer.

But no one shaped nuclear history more than Leo Szilard did.

Why He Still Matters

Leo’s legacy resonates more than ever before. In an age when multiple nations possess nuclear weapons—and where more countries are seeking to join them—we must painstakingly prioritize what should be done over what can be done. Szilard reminds us that scientific inventions can improve our lives, but must always be tempered by ethical constraints.

His legacy provides grist for policy debates today:

• Secrecy vs. Transparency: Szilard feared what unchecked secrecy could mean. In a world of classified laboratories and covert weapons programs, his call for open dialogue is more relevant than ever.

• Power vs. Principle: Szilard argued that technological power without moral constraint leads to catastrophe. The greater the power, the greater the catastrophe. Which seems obvious, of course, yet here we are.

• Science and Society: Szilard understood that technology doesn’t exist in a vacuum. Scientists are not just inventors – the work they do today largely determines what civilization will look like tomorrow.

The War Criminal

In 1960, Szilard published a short story titled My Trial as a War Criminal. In his usual blend of irony and insight, Leo imagined himself criminally charged for helping create nuclear weapons. But his story made people think: What if scientists were held responsible not just for their discoveries, but for their consequences as well? What if we all were?

Szilard suffered a fatal heart attack in 1964, but Leo’s voice still resonates through his work, his writings, and his shining example of ethical behavior.

The Path Forward

At Our Planet Project Foundation, we believe Leo Szilard’s story is more than just interesting history. It’s our guide to a survivable future. Szilard helped unlock the power of the atom, then spent the rest of his life urging the world to carefully consider how that power was used.

Nuclear weapons exist because we choose to tolerate them. But Leo Szilard believed, as we believe, that informed and organized citizens can lead the way toward nuclear disarmament. After all, the future belongs less to those who choose to build nuclear weapons, and more to those who choose not to.