The most significant work was done by Eugene Wigner (who came from Hungary), George Gamow (Russia), Felix Bloch (Switzerland), HansBethe (Germany), Edward Teller (Hungary), and Victor Weisskopf (Austria).
The letter was drafted by HansBethe, who headed the group that designed the world's first nuclear bombs and who has served as a senior adviser to previous presidents on nuclear and anti-ballistic missile issues.