It was 1 a.m. on July 16, 1945, when J. Robert Oppenheimer met with an Army lieutenant general, Leslie Groves, in the parched landscape of Jornada del Muerto — Dead Man’s Journey — a remote desert in New Mexico.
A group of engineers and physicists was about to detonate an atomic device packed with 13 pounds of plutonium, a nuclear weapon that the government hoped would bring an end to World War II.
Mr. Oppenheimer, who was tasked with designing an atomic bomb for the Manhattan Project, had not slept.
It was the first nuclear test in history.
Less than a month later, the United States would drop a nearly identical weapon on the city of Nagasaki in Japan.
The bomb, named Fat Man, fell three days after Americans dropped a uranium bomb, called Little Boy, on Hiroshima. Both weapons immediately killed tens of thousands of Japanese people and forced Japan’s surrender on Aug. 14, bringing an abrupt end to the war.
Since the Trinity test 75 years ago, at least eight countries have conducted more than 2,000 nuclear bomb tests, said Jenifer Mackby, a senior fellow at the Federation of American Scientists. More than half of those tests have been conducted by the United States, a legacy of the Trinity explosion, as the United States and several other countries have continued to refuse to ratify the treaty prohibiting nuclear weapon test explosions.
“You could say it unleashed the nuclear age, really,” Ms. Mackby said. “It unleashed a whole new class of destruction.”
Many of the scientists who witnessed the blast quickly realized the “foul and awesome” power they had set free, according to historians.
Mr. Oppenheimer said a Hindu scripture ran through his mind at the sight of the explosion: “Now I am become Death, the destroyer of worlds.”
Kenneth T. Bainbridge, the test director, was less poetic.
“Now we are all sons of bitches,” he said.
The top secret test was heard and seen for miles.
The goal of the test was to see if the military could harness plutonium into a weapon that would destroy whole cities, said Alex Wellerstein, a science historian at the Stevens Institute of Technology in Hoboken, N.J., who studies the history of nuclear weapons.
The effects of radiation were not well understood by most scientists on the project at the time, according to historians, and the preparations that were made to keep civilians safe reflected that ignorance.
They placed crude monitors around the small towns within 40 miles of the testing site. A scientist who was seven months pregnant and her husband, who was also a scientist, were sent to a motel in one of the towns with a Geiger counter, a device used to detect radioactive emissions, to measure the radiation. If the needle hit a certain mark, she was instructed to alert officials so that they could evacuate the town, Professor Wellerstein said.
Officials did not warn any of the residents — many of them ranchers, Navajos, Mexican settlers and their descendants who raised cattle and drank water from cisterns — about the test. Should anyone ask about the blast, officials had proposed several cover stories, including telling the public that a remote ammunitions depot had exploded, Professor Wellerstein said.
“They took some effort” to protect the public, he said. “Would we consider it adequate today? No, not at all. It’s not considered adequate to set off a nuclear bomb, not tell anyone about it and set up a pregnant scientist in a motel with a Geiger counter to monitor radiation.”
The blast stunned bewildered residents of the small towns within a 50-mile radius of the site.
“It produced more light and heat than the sun,” said Tina Cordova, a founder of the Tularosa Basin Downwinders Consortium, which has urged the government to conduct more research about the aftermath of the blast and to compensate the affected communities.
Based on census data at the time, the consortium estimates there were tens of thousands of people living within a 50-mile radius of the blast, Ms. Cordova said.
“Ash fell for days afterward in the landscape and in every direction and in amazing quantities,” she said.
Warnings went unseen and ignored.
The day after the blast, Leo Szilard, a Hungarian physicist who worked on the Manhattan Project, sent a petition signed by 70 scientists to President Harry S. Truman, urging him to give Japan a chance to surrender before dropping the bombs.
“Thus a nation which sets the precedent of using these newly liberated forces of nature for purposes of destruction may have to bear the responsibility of opening the door to an era of devastation on an unimaginable scale,” the petition cautioned.
It was not the first plea to reconsider using a nuclear bomb to end the war.
A month before the test, a committee, which included Dr. Szilard and was headed by the German scientist James Franck, issued the Franck Report, urging the United States to first demonstrate the power of the weapons to members of the United Nations.
Such a demonstration, the report said, would say to the world: “You see what weapon we had but did not use. We are ready to renounce its use in the future and to join other nations in working out adequate supervision of the use of this nuclear weapon.”
Mr. Truman did not see Dr. Szilard’s petition and he most likely did not see the Franck Report, said Steve Olson, who has written a book about the development of plutonium at the Hanford nuclear reservation in southeastern Washington State.
“It’s very hard to conceive of a set of developments in 1945 that would have avoided dropping those bombs,” Mr. Olson said. “Truman wanted to end the war as quickly as possible.”
The United States wanted “unconditional surrender” from Japan, he said. “Government leaders thought that was going to require a psychological shock.”
There were repercussions and regret.
The bombs in Nagasaki and Hiroshima are believed to have killed up to about 200,000 people, with many of those victims succumbing to radiation poisoning in the weeks that followed.
Scientists “were totally shocked when the Japanese reported radiation sickness at Nagasaki,” said Professor Wellerstein, who has written about what the United States knew about the long-term consequences of using the weapons.
While scientists were concerned about the possible effects of radiation on their own staff, they showed little interest in calculating what that damage could be for the Japanese, Professor Wellerstein said.
He added that they expected “the blast and fire effects of the atomic bomb would greatly overshadow any radiation casualties.”
The destruction of the cities would haunt Mr. Oppenheimer, who worried he had set a course for a future apocalypse.
“Mr. President, I feel I have blood on my hands,” he said to Mr. Truman later that year.
The true effects of the test on the people who lived near the test site remain unclear.
The government never conducted a full investigation into the effects of the radiation, even after the communities downwind of the blast saw an unusual spike in infant deaths in the months after the explosion, said Joseph J. Shonka, a scientist and one of the authors of a 2010 study about the effects of nuclear testing for the Centers for Disease Control and Prevention.
“The Trinity downwinders have not been treated in either a fair or a just manner,” he said.
Ms. Cordova, who grew up in Tularosa, N.M., said cancer had been pervasive in the towns near the Trinity test site, where everyone can name someone who died of the disease.
“We know that the government basically walked away and has taken no responsibility for the suffering and the dying,” said Ms. Cordova, who has survived thyroid cancer and has several relatives who died of various forms of cancer.
Members of Congress from New Mexico have introduced legislation that would expand the Radiation Exposure Compensation Act, which compensates uranium miners and people who lived downwind from nuclear testing sites, to include the residents who lived around Trinity.
In 2014, the National Cancer Institute began interviewing people who lived in the towns near the testing site to try and document the effects of the blast. The institute said it anticipated publishing the results “within the next few months.”