Edward Teller was a Hungarian-American physicist who made significant contributions to nuclear weapons research and development. He helped develop the first hydrogen bomb through his work with Ulam on the Teller-Ulam design. Teller had regrets about not persuading scientists to demonstrate the atomic bomb rather than use it on Hiroshima and Nagasaki. He testified against Oppenheimer at his security hearing, damaging their relationship. Teller continued his nuclear work as director of Lawrence Livermore National Lab and advocated for nuclear testing and strategic defense initiatives throughout his career.