March 14 marks 140 years since the birth of Albert Einstein, the renown physicist, Nobel Prize winner, and father of the atomic age.

Or was he?

Although the first two labels are most certainly correct, the third moniker–though widely-held–may not be entirely correct. Especially if you ask Einstein himself.

Born in Ulm, Germany in 1879 to non-practicing Jews, Einstein excelled at both math and physics from an early age, teaching himself both algebra and geometry at only twelve years of age and calculus by fourteen. In addition to advanced academic powers, Einstein also developed deeply held political and social beliefs early in life, embracing secular humanism, more agnostic views over atheism, and staunchly pacifist leanings. In fact, he renounced his German citizenship in 1896 in order to avoid state-mandated military service.

Shortly before Hitler came to power in 1933, Einstein left Europe for good and took a position at the Institute for Advanced Study in Princeton, New Jersey. Already a celebrity in part due to his 1922 Nobel Prize in Physics as well as previous trip to the U.S. in which he traveled to the White House, hobnobbed with fellow pacifist Charlie Chaplin, and was given the keys to New York City by Mayor Jimmy Walker, he found refuge in the country and ultimately applied for U.S. citizenship in 1935.

Although his citizenship was granted, his outspoken pacifism and left-leaning politics did not go unnoticed; he was immediately put on the FBI’s radar and was routinely surveilled for over twenty-two years. Upon his death in 1955, the government file on him  surpassed 1,800 pages.

So it may be surprising to learn that a pacifist devout and outspoken enough to land himself on a government watch list would, in the late 1930’s, find himself at the helm of a movement to bring forth unprecedented weaponry to the United States’ arsenal.

In December 1938, German chemists Otto Hahn and Fritz Strassman, buoyed by Einstein’s most famous equation E=mc2, which had made the splitting of atoms theoretically possible, succeeded in splitting a uranium atom in two. The discovery was announced at a physicist conference at George Washington University on January 27, 1939 and soon scientists from around the globe were attempting to achieve the same process on their own. Almost overnight, the enormous energy that had bound an atom together became available for man to harness as he wished.

Meanwhile, across Europe, Hitler’s reign of terror was beginning. The world would soon be at war.

What happened next is a matter lost to the he said/he said version of history. What is  known for sure is that on August 2, 1939, a letter signed by Einstein was delivered to President Roosevelt, imploring him to consider the implications of fission in terms of a bomb stronger than anything the world had ever known. Einstein later claimed his colleague Leo Szilard wrote the letter and he himself merely signed his name. Szilard, on the other hand, claimed Einstein was deeply involved in writing the letter. According to his account, it was Szilard who convinced Einstein about the possibility of fission being weaponized, to which Einstein replied, “I never thought of that!” The idea caused him such a panic, especially when combined with the rumors of German scientists already experimenting with such a device, that he immediately dictated a draft in German which was later translated to English.

The letter, in part, encouraged “quick action on the part of the Administration…” because “…it may become possible to set up a nuclear chain reaction in a large mass of uranium…this new phenomenon would also lead to the construction of bombs, and it is conceivable–though much less certain–that extremely powerful bombs of a new type may thus be constructed.” He urged Roosevelt to “give particular attention to the problem of securing a supply of uranium ore for the United States [and] to speed up the experimental work…by providing funds.”

Roosevelt responded by saying he had “convened a board to thoroughly investigate the possibilities of your suggestion regarding the element of uranium.” The board’s progress, however, was too slow for Einstein’s liking and, in March 1940, he wrote a second letter, warning him that, “since the outbreak of war, interest in uranium has intensified in Germany.” He informed him of the process made my Szilard on uranium chain reactions and urged him to devote more resources toward the science.

Despite this, it wasn’t until 1942 that the idea behind atomic weapons really began to take off, when the program was handed over to the U.S. Army under the code name “Manhattan.” Einstein, desperate to help, volunteered for the project, but was ultimately denied the security clearance needed. Scientists were even forbidden to consult with him on project matters aside from one small issue–the problem involving separation of isotopes that shared chemical traits, which Einstein solved in less than two days.

But despite this, or perhaps because of it, Einstein was already beginning to have doubts. In December 1944, he wrote to physicist Niels Bohr, saying “…when the war is over, then there will be in all countries a pursuit of secret war preparations with technological means which will lead inevitably to preventative wars and to destruction even more terrible than the present destruction of life.”

Work, however, progressed without him and, on August 6, 1945, the world’s first atomic weapon was dropped on Hiroshima, Japan, followed soon after by another on Nagasaki. Einstein, upon hearing the news, supposedly muttered the words, “Woe is me.”

Although the United States celebrated the weapon and the war’s subsequent end, Einstein himself was filled with regret at the effects of the weapon whose creation he had set in motion. It wasn’t just the sheer amount of devastation that bothered him; it was the fact the weapon was used at all. Einstein had put his support behind a bomb only due to a threat of a similar weapon in the hands of the Nazi’s. The bombing of Japan, however, came three months after German’s surrender, when the Nazi threat had long passed, and the explosion had occurred without warning, an aggressive move rather than a last resort.

Immediately after the bomb, he began distancing himself from the project. He maintained that his work in physics did not provide a map for fission but only explained the energy released during it. “I do not consider myself the father of atomic energy,” he said. “My part in it was quite indirect.”

This line, no matter how oft repeated, did little to quell his remorse or stifle the praises of the general public. In an interview with Newsweek, he tried to make himself even more clear by saying “Had I known that the Germans would not succeed in developing an atomic bomb, I would have done nothing.”

Einstein spent the remaining years of his life fighting against the very thing he claimed to have had a very small part in. In 1955, he added his name to the Russell-Einstein Manifesto, which called for a conference where scientists from all nations could assess the dangers of nuclear weapons. It was hoped that, through this, world leaders would understand the risk of “playing with fire” and seek more peaceful resolutions to global conflicts.

Directly or indirectly, big or small, Einstein’s role in the development of atomic weapons weighed heavily enough upon his shoulders to bring about a lifetime of rallying against their use. To him, however, the efforts were too little too late. Just months before his death, he told friend and renown chemist and peace activist Linus Pauling that “I made one great mistake in my life…when I signed the letter to President Roosevelt recommending that atom bombs be made.”