Washington communicated with field commanders by classified cable. No one working in those code rooms would be cleared for knowledge of Manhattan. So, the only way to communicate way by sending briefers overseas to personally brief the person in secret.
I read somewhere that Marshall discussed the issue of dropping the bomb with Ike and Bradley when visiting them but it was not an authoritative source. I don't know why they would be in the loop. No need to know there, but Marshall did seem to value their counsel.
Most of the classified briefings were done by Army engineers assigned to the Manhattan District, and were able to convey what the bombs were all about. I wonder if Marshall was able to convey the same level of understanding to Ike and Bradley. All three of them were of a generation where atomic energy was not generally known.
Tibbets and his Manhattan consultants thought LeMay didn't really understand what the bomb was. Most of that generation looked at it as just a really big bomb, and not something totally new and world-changing. Hitler never "got" nuclear physics, referring to it as "Jewish science" so he didn't put any resources into Heisenberg's research.
Kind of like how I don't get Facebook, Twitter, Instagram, and have trouble using Google Drive and Google Documents to share and edit as a group.
