From 1942 to 1945, the Manhattan Project had operated in secret to develop a weapon more fearsome than anything the world had ever seen. Now, in mid-July, they were ready — or at least, they hoped so.

The Trinity test—the detonation of the world’s first nuclear bomb—was conducted 80 years ago as part of the Manhattan Project. Two years ago, the film Oppenheimer premiered in theaters and became an ...