Robert Oppenheimer

You mean Rene Robert Cavalier Sieur de La Salle. He was famous for exploring up and down the Mississippi River for France

It's pretty convoluted, but the short answer is that they thought he might be a Soviet spy.

Oppenheimer had, when younger, had some friends and acquaintances who were communists. He had drifted apart from most of these, but was still on fairly good terms with at least one of them. A Soviet agent approached his friend, and that friend spoke with Oppenheimer about it. Very briefly and casually; it was something along the lines of "So-and-so was wondering if there anything you can do to make sure the Soviets are kept in the loop, perhaps unofficially?" and Oppenheimer's response was something along the lines of "No, that's not how to do these things."

The problem was that when Oppenheimer reported this to his superiors, he didn't report it accurately and tried to hide his own involvement and that of his friend. He just said that "some people" had been approached on behalf of the Soviets. So the US started a big spy hunt for these Soviet agents, and Oppenheimer's responses to their questions weren't adding up. In fact, he wound up making things a lot worse for his friend than was actually justified, because the Government already suspected both Oppenheimer and his friend based on their (known) past association with communists, and Oppenheimer trying to cover it up made it look as if their involvement had been much more sinister than it really was.

Xbox

1945... Secretary of War Stimson, visiting my headquarters in Germany, informed me that our government was preparing to drop an atomic bomb on Japan. I was one of those who felt that there were a number of cogent reasons to question the wisdom of such an act. ...the Secretary, upon giving me the news of the successful bomb test in New Mexico, and of the plan for using it, asked for my reaction, apparently expecting a vigorous assent.

"During his recitation of the relevant facts, I had been conscious of a feeling of depression and so I voiced to him my grave misgivings, first on the basis of my belief that Japan was already defeated and that dropping the bomb was completely unnecessary, and secondly because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment was, I thought, no longer mandatory as a measure to save American lives. It was my belief that Japan was, at that very moment, seeking some way to surrender with a minimum loss of 'face'. The Secretary was deeply perturbed by my attitude..."

- Dwight Eisenhower, Mandate For Change, pg. 380

Yes, J. Robert Oppenheimer, an American theoretical physicist and one of the key figures in the development of the atomic bomb during World War II, had his security clearance revoked in 1954, which effectively stripped him of his ability to work on classified projects. While his security clearance was revoked, his citizenship was not taken away.

Robert J. Meder died in 1943.

The primary reason was that Edward Teller hated him. A secondary reason was Joseph McCarthy's communist witch hunts at the time and that fact that both Robert Oppenheimer's wife Kitty and brother Frank had been registered members of the communist party during the 1930s (like many in universities at the time).

Robert J. Gamble died on 1924-09-22.

Like most of the scientists on the Manhattan Project, the threat (or fear) that the Nazi's were developing an atomic weapon, basically compelled them (and the Army) to proceed with the development.